1. Multicellularity
Ponder this one in the bath. Chances are you've just scrubbed your back with a choice example of one of evolution's greatest inventions. Or at least, a good plastic copy.
Sponges are a key example of multicellular life, an innovation that transformed living things from solitary cells into fantastically complex bodies. It was such a great move, it evolved at least 16 different times. Animals, land plants, fungi and algae all joined in.
Cells have been joining forces for billions of years. Even bacteria can do it, forming complex colonies with a three-dimensional structure and some division of labour. But hundreds of millions of years ago, eukaryotes - more complex cells that package up their DNA in a nucleus - took things to a new level. They formed permanent colonies in which certain cells dedicated themselves to different tasks, such as nutrition or excretion, and whose behaviour was well coordinated.
Eukaryotes could make this leap because they had already evolved many of the necessary attributes for other purposes. Many single-celled eukaryotes can specialise or "differentiate" into cell types, dedicated to specific tasks such as mating with another cell. They sense their environment with chemical signalling systems, some of which are similar to those multicellular organisms use to coordinate their cells' behaviour. And they may detect and capture their prey with the same kind of sticky surface molecules that hold cells together in animals and other multicellular organisms.
So what started it? One idea is that clumping together helped cells avoid being eaten by making them too much of a mouthful for single-celled predators. Another is that single cells are often constrained in what they can do - for example, most cannot grow flagella to move and also divide at the same time. But a colony can both move and contain dividing cells if each cell in it takes its turn.
Researchers are now trying to reconstruct the biology of the first multicellular creatures by studying the genomes of their nearest living relatives. "We're trying to peer back hundreds of millions of years," says Nicole King, a molecular biologist at the University of California, Berkeley. She and her team are studying single-celled protozoans called choanoflagellates to understand how animals came to evolve from them some 600 million years ago. Choanoflagellates and sponges - the only surviving witnesses to this step - share a common ancestor and King has found that choanoflagellates have a surprising number of equivalents to the signalling and cell-adhesion molecules unique to animals.
Yet bigger and more complex isn't necessarily better. As King points out, unicellular life still vastly outnumbers multicellular life in terms of both biomass and species numbers. "So you could say unicellular life is the most successful, but that multicellular life is the most beautiful and dramatic."
Claire Ainsworth. Back to top
2. The eye
THEY appeared in an evolutionary blink and changed the rules of life forever. Before eyes, life was gentler and tamer, dominated by sluggish soft-bodied worms lolling around in the sea. The invention of the eye ushered in a more brutal and competitive world. Vision made it possible for animals to become active hunters, and sparked an evolutionary arms race that transformed the planet.
The first eyes appeared about 543 million years ago - the very beginning of the Cambrian period - in a group of trilobites called the Redlichia. Their eyes were compound, similar to those of modern insects, and probably evolved from light-sensitive pits. And their appearance in the fossil record is strikingly sudden - trilobite ancestors from 544 million years ago don't have eyes.
So what happened in that magic million years? Surely eyes are just too complex to appear all of a sudden? Not so, according to Dan-Eric Nilsson of Lund University in Sweden. He has calculated that it would take only half a million years for a patch of light-sensitive cells to evolve into a compound eye.
“Eyes sparked an evolutionary arms race that transformed the planet”
That's not to say the difference was trivial. Patches of photosensitive cells were probably common long before the Cambrian, allowing early animals to detect light and sense what direction it was coming from. Such rudimentary sense organs are still used by jellyfish, flatworms and other obscure and primitive groups, and are clearly better than nothing. But they are not eyes. A true eye needs something extra - a lens that can focus light to form an image. "If you suddenly obtain a lens, the effectiveness goes from about 1 per cent to 100 per cent," says Andrew Parker, a zoologist at the University of Oxford.
Trilobites weren't the only animals to stumble across this invention. Biologists believe that eyes could have evolved independently on many occasions, though genetic evidence suggests one ancestor for all eyes. But either way, trilobites were the first.
And what a difference it made. In the sightless world of the early Cambrian, vision was tantamount to a super-power. Trilobites' eyes allowed them to become the first active predators, able to seek out and chase down food like no animal before them. And, unsurprisingly, their prey counter-evolved. Just a few million years later, eyes were commonplace and animals were more active, bristling with defensive armour. This burst of evolutionary innovation is what we now know as the Cambrian explosion.
However, sight is not universal. Of 37 phyla of multicellular animals, only six have evolved it, so it might not look like such a great invention after all - until you stop to think. The six phyla that have vision (including our own, chordates, plus arthropods and molluscs) are the most abundant, widespread and successful animals on the planet.
Graham Lawton. Back to top
3. The brain
Brains are often seen as a crowning achievement of evolution - bestowing the ultimate human traits such as language, intelligence and consciousness. But before all that, the evolution of brains did something just as striking: it lifted life beyond vegetation. Brains provided, for the first time, a way for organisms to deal with environmental change on a timescale shorter than generations.
A nervous system allows two extremely useful things to happen: movement and memory. If you're a plant and your food source disappears, that's just tough. But if you have a nervous system that can control muscles, then you can actually move around and seek out food, sex and shelter.
The simplest nervous systems are just ring-like circuits in cnidarians - the jellyfish, urchins and anemones. These might not be terribly smart, but they can still find the things they need and interact with the world in a far more sophisticated way than plants manage.
The next evolutionary step, which probably happened in flatworms in the Cambrian, was to add some sort of control system to give the movements more purpose. This sort of primitive brain is simply a bit of extra wiring that helps organise the networks.
Armed with this, finding food would have been the top priority the earliest water-dwelling creatures. Organisms need to sort out nutritious from toxic food, and the brain helps them do that. Sure enough, look at any animal and you will find the brain is always near the mouth. In some of the most primitive invertebrates, the oesophagus actually passes right through the brain.
With brains come senses, to detect whether the world is good or bad, and a memory. Together, these let the animal monitor in real time whether things are getting better or worse. This in turn allows a simple system of prediction and reward. Even animals with really simple brains - insects, slugs or flatworms - can use their experiences to predict what might be the best thing to do or eat next, and have a system of reward that reinforces good choices.
The more complex functions of the human brain - social interaction, decision-making and empathy, for example - seem to have evolved from these basic systems controlling food intake. The sensations that control what we decide to eat became the intuitive decisions we call gut instincts. The most highly developed parts of the human frontal cortex that deal with decisions and social interactions are right next to the parts that control taste and smell and movements of the mouth, tongue and gut. There is a reason we kiss potential mates - it's the most primitive way we know to check something out.
Helen Phillips. Back to top
4. Language
As far as humans are concerned, language has got to be the ultimate evolutionary innovation. It is central to most of what makes us special, from consciousness, empathy and mental time travel to symbolism, spirituality and morality. Language may be a defining factor of our species, but just how important is it in the evolutionary scheme of things?
A decade ago, John Maynard Smith, then emeritus professor of biology at the University of Sussex, UK, and Eors Szathmary from the Institute of Advanced Study in Budapest, Hungary, published The Major Transitions in Evolution, their description of life's great leaps forward. They identified these crucial steps as innovations in the way information was organised and transmitted from one generation to the next - starting with the origin of life itself and ending with language.
Exactly how our ancestors took this leap is possibly the hardest problem in science, Szathmary says. He points out that complex language - language with syntax and grammar, which builds up meaning through a hierarchical arrangement of subordinate clauses - evolved just once. Only human brains are able to produce language, and, contrary to popular belief, this ability is not confined to specialised regions in the brain such as Broca's and Wernicke's areas. If these are damaged others can take over. Szathmary likens language to an amoeba, and the human brain to the habitat in which it can thrive. "A surprisingly large part of our brain can sustain language," he says.
But that raises the question of why this language amoeba doesn't colonise the brains of other animals, especially primates. Szathmary is convinced the answer lies in neural networks unique to humans that allow us to perform the complex hierarchical processing required for grammatical language. These networks are shaped both by our genes and by experience. The first gene associated with language, FOXP2, was identified in 2001, and others will surely follow.
So why don't our close evolutionary relatives, chimps and other primates, have similar abilities? The answer, recent analysis seems to suggest, lies in the fact that while humans and chimps have many genes in common, the versions expressed in human brains are more active than those in chimps. What's more, the brains of newborn humans are far less developed than those of newborn chimps, which means that our neural networks are shaped over many years of development immersed in a linguistic environment.
In a sense, language is the last word in biological evolution. That's because this particular evolutionary innovation allows those who possess it to move beyond the realms of the purely biological. With language, our ancestors were able to create their own environment - we now call it culture - and adapt to it without the need for genetic changes.
Kate Douglas. Back to top
5. Photosynthesis
FEW innovations have had such profound consequences for life as the ability to capture energy from sunlight. Photosynthesis has literally altered the planet's face, transforming the atmosphere and cocooning Earth in a protective shield against lethal radiation.
Without photosynthesis, there would be little oxygen in the atmosphere, and no plants or animals - just microbes scratching a meagre existence from a primordial soup of minerals and carbon dioxide. It freed life from these constraints and the oxygen it generated set the stage for the emergence of complex life.
Before photosynthesis, life consisted of single-celled microbes whose sources of energy were chemicals such as sulphur, iron and methane. Then, around 3.5 billion years ago, or perhaps earlier, a group of microbes developed the ability to capture energy from sunlight to help make the carbohydrates they needed for growth and fuel. It is unclear how they achieved this feat, but genetic studies suggest that the light-harvesting apparatus evolved from a protein with the job of transferring energy between molecules. Photosynthesis had arrived.
But this early version of the process didn't make oxygen. It used hydrogen sulphide and carbon dioxide as its starting ingredients, generating carbohydrates and sulphur as end products. Some time later - just when is uncertain - a new type of photosynthesis evolved that used a different resource, water, generating oxygen as a by-product.
In those early days, oxygen was poisonous to life. But it built up in the atmosphere until some microbes evolved mechanisms to tolerate it, and eventually hit on ways to use it as an energy source. That was a pretty important discovery too: using oxygen to burn carbohydrates for energy is 18 times as efficient as doing it without oxygen.
Life on Earth became high-powered at this point, setting the scene for the development of complex, multicellular life forms - including plants, which "borrowed" their photosynthetic apparatus from photosynthetic bacteria called cyanobacteria. Today, directly or indirectly, photosynthesis produces virtually all of the energy used by life on Earth.
As well as providing an efficient means to burn fuel, oxygen made by photosynthesis helps protect life. Earth is under constant bombardment from lethal UV radiation streaming out from the sun. A by-product of our oxygenated atmosphere is a layer of ozone extending 20 to 60 kilometres above Earth's surface, which filters out most of the harmful UV. This protective umbrella allowed life to escape from the sanctuary of the ocean and colonise dry land.
“It has altered the atmosphere and cocooned Earth in a protective shield”
Now, virtually every biochemical process on the planet is ultimately dependent on an input of solar energy. Take a deep breath and thank those primordial oxygen-hating microbes for their biochemical inventiveness.
Alison George. Back to top
6. Sex
Birds do it, bees do it - for the vast majority of species, sexual reproduction is the only option. And it is responsible for some of the most impressive biological spectacles on the planet, from mass spawnings of coral so vast that they are visible from space, to elaborate sexual displays such as the dance of the bower bird, the antlers of a stag and - according to some biologists - poetry, music and art. Sex may even be responsible for keeping life itself going: species that give it up almost always go extinct within a few hundred generations.
Important as sex is, however, biologists are still arguing over how it evolved - and why it hasn't un-evolved. That's because, on the face of it, sex looks like a losing strategy.
Evolution ought to favour asexual reproduction for two reasons. First, in the battle for resources, asexual species should be able to outcompete sexual ones hands down. And secondly, because sperm and eggs contain only half of each parent's set of genes, an organism that uses sexual reproduction only gets 50 per cent of its genes into the next generation. Asexuals are guaranteed to pass on 100 per cent.
Clearly, though, there is something wrong with this line of reasoning. It's true that many species, including insects, lizards and plants, do fine without sex, at least for a while. But they are vastly outnumbered by sexual ones.
The enduring success of sex is usually put down to the fact that it shuffles the genetic pack, introducing variation and allowing harmful mutations to be purged (mutations are what eventually snuffs out most asexual species). Variation is important because it allows life to respond to changing environments, including interactions with predators, prey and - particularly - parasites. Reproducing asexually is sometimes compared to buying 100 tickets in a raffle, all with the same number. Far better to have only 50 tickets, each with a different number.
However useful sex may be now that we've got it, that doesn't tell us anything about how it got started. It could have been something as mundane as DNA repair. Single-celled, asexual organisms may have developed the habit of periodically doubling up their genetic material, then halving it again. This would have allowed them to repair any DNA damage by switching in the spare set. A similar exchange of DNA still happens during the production of eggs and sperm.
Parasites are also in the frame. Parasitic lengths of DNA known as transposons reproduce by inserting copies of themselves into the cell's normal genetic material. Imagine a transposon within a single-celled organism acquiring a mutation that happens to cause its host cell to periodically fuse with other cells before dividing again. The transposon for this primitive form of sex would be able to spread horizontally between many different cells. Once it arose in a population, parasitic sex would catch on pretty quickly.
Clare Wilson. Back to top
7. Death
Could evolution have brought the Grim Reaper into being? Yes, indeed. Not in all his guises, of course - living things have always died because of mishaps such as starvation or injury. But there's another sort of death in which cells - and perhaps, controversially, even whole organisms - choose annihilation because of the benefits it brings to some greater whole. In other words, death is an evolutionary strategy.
This is most obvious in the many varieties of programmed cell death or apoptosis, a self-destruct mechanism found in every multicellular organism. Your hand has five fingers because the cells that used to live between them died when you were an embryo. Embryos as tiny as 8 to 16 cells - just 3 or 4 cell divisions after the fertilised egg - depend on cell death: block apoptosis and development goes awry. Were it not for death, we would not even be born.
Even as adults we could not live without death. Without apoptosis we would all be overrun by cancer. Your cells are constantly racking up mutations that threaten to make your tightly controlled cell division run amok. But surveillance systems - such as the one involving the p53 protein, called the "guardian of the genome" (New Scientist, 18 December 2004, p 38) - detect almost all such errors and direct the affected cells to commit suicide.
Programmed cell death plays a central role in everyday life too. It ensures a constant turnover of cells in the gut lining and generates our skin's protective outer layer of dead cells. When the immune system has finished wiping out an infection, the now-redundant white blood cells commit suicide in an orderly fashion to allow the inflammation to wind down. And plants use cell death as part of a scorched-earth defence against pathogens, walling off the infected area and then killing off all the cells within.
It is easy to see how an organism can benefit from sacrificing a few cells. But evolution may also have had a hand in shaping the death of whole organisms. The cells of all higher organisms begin to age, or senesce, after just a few dozen cell divisions, ultimately leading to the death of the organism itself. In part that is one more protection against uncontrolled growth. But one controversial theory suggests this is part of an inbuilt genetic ageing program that sets an upper limit on all our lifespans (New Scientist, 19 April 2004, p 26).
Most evolutionary biologists reject the idea of an innate "death program". After all, they point out, animals die of old age in many different ways, not by one single route as apoptotic cells do. Instead, they view senescence as a sort of evolutionary junkyard: natural selection has little reason to get rid of flaws that appear late in life, since few individuals are lucky enough to make it to old age. But now that people routinely survive well past reproductive age, we suffer the invention evolution never meant us to find: death by old age.
Bob Holmes. Back to top
8. Parasitism
The name is synonymous with stealing, cheating and stealthy evil. But the age-old battle between parasites and their hosts is one of the most powerful driving forces in evolution. Without its plunderers and freeloaders, life would simply not be the same.
From viruses to tapeworms, barnacles to birds, parasites are among the most successful organisms on the planet, taking merciless advantage of every known creature. Take the tapeworm. This streamlined parasite is little more than gonads and a head full of hooks, having dispensed with a gut in favour of bathing in the nutrient-rich depths of its host's digestive system. In its average 18-year lifespan, a human tapeworm can generate 10 billion eggs.
Many parasites, such as the small liver fluke, have also mastered the art of manipulating their host's behaviour. Ants whose brains are infected with a juvenile fluke feel compelled to climb to the tops of grass blades, where they are more likely to be eaten by the fluke's ultimate host, a sheep.
"They are really disgusting, but man, are they good at what they do," says Daniel Simberloff, an ecologist at the University of Tennessee and translator of the popular French text The Art of Being a Parasite. "Evolution is in large part probably driven by parasites. It is the main hypothesis for the continuation of sexual reproduction. How much more important can you get?"
The parasites that have had arguably the biggest effect on evolution are the smallest. Bacteria, protozoans and viruses can shape the evolution of their hosts because only the hardiest will survive infection. And humans are no exception: the genes for several inherited conditions protect against infectious disease when inherited in a single dose. For example, one copy of the gene for sickle cell anaemia protects against malaria. And it is still happening today. HIV and TB, for instance, are driving evolutionary change in parts of our genome, such as the immune-system genes (New Scientist, 22 November 2003, p 44).
Hosts can influence the evolution of their parasites too. For example, diseases which require human-to-human contact for transmission often evolve to be less deadly, ensuring a person will at least live long enough to pass it on.
Parasites can also drive the evolution at a more basic level. Parasitic lengths of DNA called transposons, which can cut and paste themselves all over the genome, can be transformed into new genes or encourage the mutation and shuffling of DNA that fuels genetic variation. They have even been implicated in the origins of sex, as they may have driven selection for cell fusion and gamete formation (see opposite).
Anna Gosline. Back to top
9. Superorganism
LARGE numbers of individuals living together in harmony, achieving a better life by dividing their workload and sharing the fruits of their labours. We call this blissful state utopia, and have been striving to achieve it for at least as long as recorded history. Alas, our efforts so far have been in vain. Evolution, however, has made a rather better job of it.
Take the Portuguese man-of-war. It may look like just another jellyfish blob floating on the high seas, but zoom in with a microscope and you see that what seemed like one tentacled individual is in fact a colony of single-celled organisms. These "siphanophores" have got division of labour down to a fine art. Some are specialised for locomotion, some for feeding, some for distributing nutrients.
This communal existence brings major advantages. It allows the constituent organisms, which would otherwise be rooted to the sea floor, to swim free. And together they are better able to defend themselves against predators, cope with environmental stress, and colonise new territory. Portuguese man-of-war jellyfish are truly superorganisms.
With benefits like these on offer, it should come as no surprise that colonial living has evolved many times. Except that it does come with one big drawback, as the case of the slime bacteria, or myxobacteria, illustrates. These microbes are perhaps the simplest colonial organisms. Under normal circumstances individual bacteria glide along on lonely slime trails. Only when certain amino acids are lacking in their environment do individuals start to aggregate. The resulting superorganism consists of a stalk topped by a fruiting body containing spores. But since only the bacteria forming the spores will get the chance of dispersal and a new life, why should the others play along? How this kind of cooperation evolved, and how cheats are prevented from taking advantage of it remains unclear for some types of colonial life.
But in one group of animals, the colonial insects, we do know what the trick is - and it's an ingenious one. Females develop from fertilised eggs, while males develop from unfertilised ones. This way of determining sex, called haplodiploidy, ensures that sisters are more closely related to each other than to their own offspring. And this means that the best chance they can give their own genes of surviving is to look after each other rather than lay eggs of their own. This is what provides the stability at the heart of the beehive and termite mound, and in many other insect colonies where haplodiploidy has evolved at least a dozen times.
True sociality, or eusociality as it is technically known, is found in all ants and termites, in the most highly organised bees and wasps, and in some other species, not all of which employ haplodiploidy. And although these mini societies need careful policing to keep cheats at bay, this is probably the closest thing on Earth to utopia.
Kate Douglas. Back to top
10. Symbiosis
Crocodiles with gleaming gums, coral reefs, orchids, fish with glow-in-the-dark lures, ants that farm, new directions for evolution. All that from swapping food - for cleaning services, for transport, for sunscreen, for shelter, and of course for other food.
Symbiosis has many definitions, but we'll take it to mean two species engaging in physically intimate, mutually beneficial dependency, almost invariably involving food. Symbiosis has triggered seismic shifts in evolution, and evolution in turn continually spawns new symbiotic relationships.
Perhaps the most pivotal couplings were the ones that turbocharged complex, or eukaryotic, cells. Eukaryotes use specialised organelles such as mitochondria and chloroplasts to extract energy from food or sunlight. These organelles were originally simpler, prokaryotic cells that the eukaryotes engulfed in an eternal symbiotic embrace. Without them life's key developments, such as increasing complexity and multicellular plants and animals, would not have happened. "There are only two things that matter in this world: respiration and photosynthesis. Eukaryotes didn't figure out either by themselves, they borrowed them from prokaryotes through symbiosis," says Geoff McFadden of the University of Melbourne, Australia.
Symbiosis has popped up so frequently during evolution that it is safe to say it's the rule, not the exception. Angler fish in the deep ocean host bioluminescent bacteria in appendages that dangle over their mouths. Smaller fish lured by the light are easy prey. At the ocean surface, coral polyps provide homes for photosynthetic algae, and swap inorganic waste products for organic carbon compounds - one reason why nutrient-poor tropical waters can support so much life. The algae also produce a chemical that absorbs ultraviolet light and protects the coral.
More than 90 per cent of plant species are thought to engage in symbiotic couplings. Orchid seeds are little more than dust, containing next to no nutrients. To germinate and grow, they digest a fungus that infects the seed. "Birds and animals and insects that are adapted to pollination and seed disposal, these are some of the greatest symbioses. Without them we wouldn't have most of our flowering plants," says Ursula Munro, an ecologist at the University of Technology in Sydney, Australia.
“Without symbiosis we wouldn't have most of our flowering plants”
Plovers pick leeches from crocodiles' teeth, offering dental hygiene in return for food. Leafcutter ants use chopped-up leaves as a fertiliser for the fungus they grow in underground chambers. The ants cannot digest the leaves but the fungus that feeds on them produces a tasty meal of sugars and starch while breaking down the toxins in the leaves. And there is not an animal out there, including us, that can survive without the bacteria that live in its gut, digesting food and producing vitamins.
Ponder this one in the bath. Chances are you've just scrubbed your back with a choice example of one of evolution's greatest inventions. Or at least, a good plastic copy.
Sponges are a key example of multicellular life, an innovation that transformed living things from solitary cells into fantastically complex bodies. It was such a great move, it evolved at least 16 different times. Animals, land plants, fungi and algae all joined in.
Cells have been joining forces for billions of years. Even bacteria can do it, forming complex colonies with a three-dimensional structure and some division of labour. But hundreds of millions of years ago, eukaryotes - more complex cells that package up their DNA in a nucleus - took things to a new level. They formed permanent colonies in which certain cells dedicated themselves to different tasks, such as nutrition or excretion, and whose behaviour was well coordinated.
Eukaryotes could make this leap because they had already evolved many of the necessary attributes for other purposes. Many single-celled eukaryotes can specialise or "differentiate" into cell types, dedicated to specific tasks such as mating with another cell. They sense their environment with chemical signalling systems, some of which are similar to those multicellular organisms use to coordinate their cells' behaviour. And they may detect and capture their prey with the same kind of sticky surface molecules that hold cells together in animals and other multicellular organisms.
So what started it? One idea is that clumping together helped cells avoid being eaten by making them too much of a mouthful for single-celled predators. Another is that single cells are often constrained in what they can do - for example, most cannot grow flagella to move and also divide at the same time. But a colony can both move and contain dividing cells if each cell in it takes its turn.
Researchers are now trying to reconstruct the biology of the first multicellular creatures by studying the genomes of their nearest living relatives. "We're trying to peer back hundreds of millions of years," says Nicole King, a molecular biologist at the University of California, Berkeley. She and her team are studying single-celled protozoans called choanoflagellates to understand how animals came to evolve from them some 600 million years ago. Choanoflagellates and sponges - the only surviving witnesses to this step - share a common ancestor and King has found that choanoflagellates have a surprising number of equivalents to the signalling and cell-adhesion molecules unique to animals.
Yet bigger and more complex isn't necessarily better. As King points out, unicellular life still vastly outnumbers multicellular life in terms of both biomass and species numbers. "So you could say unicellular life is the most successful, but that multicellular life is the most beautiful and dramatic."
Claire Ainsworth. Back to top
2. The eye
THEY appeared in an evolutionary blink and changed the rules of life forever. Before eyes, life was gentler and tamer, dominated by sluggish soft-bodied worms lolling around in the sea. The invention of the eye ushered in a more brutal and competitive world. Vision made it possible for animals to become active hunters, and sparked an evolutionary arms race that transformed the planet.
The first eyes appeared about 543 million years ago - the very beginning of the Cambrian period - in a group of trilobites called the Redlichia. Their eyes were compound, similar to those of modern insects, and probably evolved from light-sensitive pits. And their appearance in the fossil record is strikingly sudden - trilobite ancestors from 544 million years ago don't have eyes.
So what happened in that magic million years? Surely eyes are just too complex to appear all of a sudden? Not so, according to Dan-Eric Nilsson of Lund University in Sweden. He has calculated that it would take only half a million years for a patch of light-sensitive cells to evolve into a compound eye.
“Eyes sparked an evolutionary arms race that transformed the planet”
That's not to say the difference was trivial. Patches of photosensitive cells were probably common long before the Cambrian, allowing early animals to detect light and sense what direction it was coming from. Such rudimentary sense organs are still used by jellyfish, flatworms and other obscure and primitive groups, and are clearly better than nothing. But they are not eyes. A true eye needs something extra - a lens that can focus light to form an image. "If you suddenly obtain a lens, the effectiveness goes from about 1 per cent to 100 per cent," says Andrew Parker, a zoologist at the University of Oxford.
Trilobites weren't the only animals to stumble across this invention. Biologists believe that eyes could have evolved independently on many occasions, though genetic evidence suggests one ancestor for all eyes. But either way, trilobites were the first.
And what a difference it made. In the sightless world of the early Cambrian, vision was tantamount to a super-power. Trilobites' eyes allowed them to become the first active predators, able to seek out and chase down food like no animal before them. And, unsurprisingly, their prey counter-evolved. Just a few million years later, eyes were commonplace and animals were more active, bristling with defensive armour. This burst of evolutionary innovation is what we now know as the Cambrian explosion.
However, sight is not universal. Of 37 phyla of multicellular animals, only six have evolved it, so it might not look like such a great invention after all - until you stop to think. The six phyla that have vision (including our own, chordates, plus arthropods and molluscs) are the most abundant, widespread and successful animals on the planet.
Graham Lawton. Back to top
3. The brain
Brains are often seen as a crowning achievement of evolution - bestowing the ultimate human traits such as language, intelligence and consciousness. But before all that, the evolution of brains did something just as striking: it lifted life beyond vegetation. Brains provided, for the first time, a way for organisms to deal with environmental change on a timescale shorter than generations.
A nervous system allows two extremely useful things to happen: movement and memory. If you're a plant and your food source disappears, that's just tough. But if you have a nervous system that can control muscles, then you can actually move around and seek out food, sex and shelter.
The simplest nervous systems are just ring-like circuits in cnidarians - the jellyfish, urchins and anemones. These might not be terribly smart, but they can still find the things they need and interact with the world in a far more sophisticated way than plants manage.
The next evolutionary step, which probably happened in flatworms in the Cambrian, was to add some sort of control system to give the movements more purpose. This sort of primitive brain is simply a bit of extra wiring that helps organise the networks.
Armed with this, finding food would have been the top priority the earliest water-dwelling creatures. Organisms need to sort out nutritious from toxic food, and the brain helps them do that. Sure enough, look at any animal and you will find the brain is always near the mouth. In some of the most primitive invertebrates, the oesophagus actually passes right through the brain.
With brains come senses, to detect whether the world is good or bad, and a memory. Together, these let the animal monitor in real time whether things are getting better or worse. This in turn allows a simple system of prediction and reward. Even animals with really simple brains - insects, slugs or flatworms - can use their experiences to predict what might be the best thing to do or eat next, and have a system of reward that reinforces good choices.
The more complex functions of the human brain - social interaction, decision-making and empathy, for example - seem to have evolved from these basic systems controlling food intake. The sensations that control what we decide to eat became the intuitive decisions we call gut instincts. The most highly developed parts of the human frontal cortex that deal with decisions and social interactions are right next to the parts that control taste and smell and movements of the mouth, tongue and gut. There is a reason we kiss potential mates - it's the most primitive way we know to check something out.
Helen Phillips. Back to top
4. Language
As far as humans are concerned, language has got to be the ultimate evolutionary innovation. It is central to most of what makes us special, from consciousness, empathy and mental time travel to symbolism, spirituality and morality. Language may be a defining factor of our species, but just how important is it in the evolutionary scheme of things?
A decade ago, John Maynard Smith, then emeritus professor of biology at the University of Sussex, UK, and Eors Szathmary from the Institute of Advanced Study in Budapest, Hungary, published The Major Transitions in Evolution, their description of life's great leaps forward. They identified these crucial steps as innovations in the way information was organised and transmitted from one generation to the next - starting with the origin of life itself and ending with language.
Exactly how our ancestors took this leap is possibly the hardest problem in science, Szathmary says. He points out that complex language - language with syntax and grammar, which builds up meaning through a hierarchical arrangement of subordinate clauses - evolved just once. Only human brains are able to produce language, and, contrary to popular belief, this ability is not confined to specialised regions in the brain such as Broca's and Wernicke's areas. If these are damaged others can take over. Szathmary likens language to an amoeba, and the human brain to the habitat in which it can thrive. "A surprisingly large part of our brain can sustain language," he says.
But that raises the question of why this language amoeba doesn't colonise the brains of other animals, especially primates. Szathmary is convinced the answer lies in neural networks unique to humans that allow us to perform the complex hierarchical processing required for grammatical language. These networks are shaped both by our genes and by experience. The first gene associated with language, FOXP2, was identified in 2001, and others will surely follow.
So why don't our close evolutionary relatives, chimps and other primates, have similar abilities? The answer, recent analysis seems to suggest, lies in the fact that while humans and chimps have many genes in common, the versions expressed in human brains are more active than those in chimps. What's more, the brains of newborn humans are far less developed than those of newborn chimps, which means that our neural networks are shaped over many years of development immersed in a linguistic environment.
In a sense, language is the last word in biological evolution. That's because this particular evolutionary innovation allows those who possess it to move beyond the realms of the purely biological. With language, our ancestors were able to create their own environment - we now call it culture - and adapt to it without the need for genetic changes.
Kate Douglas. Back to top
5. Photosynthesis
FEW innovations have had such profound consequences for life as the ability to capture energy from sunlight. Photosynthesis has literally altered the planet's face, transforming the atmosphere and cocooning Earth in a protective shield against lethal radiation.
Without photosynthesis, there would be little oxygen in the atmosphere, and no plants or animals - just microbes scratching a meagre existence from a primordial soup of minerals and carbon dioxide. It freed life from these constraints and the oxygen it generated set the stage for the emergence of complex life.
Before photosynthesis, life consisted of single-celled microbes whose sources of energy were chemicals such as sulphur, iron and methane. Then, around 3.5 billion years ago, or perhaps earlier, a group of microbes developed the ability to capture energy from sunlight to help make the carbohydrates they needed for growth and fuel. It is unclear how they achieved this feat, but genetic studies suggest that the light-harvesting apparatus evolved from a protein with the job of transferring energy between molecules. Photosynthesis had arrived.
But this early version of the process didn't make oxygen. It used hydrogen sulphide and carbon dioxide as its starting ingredients, generating carbohydrates and sulphur as end products. Some time later - just when is uncertain - a new type of photosynthesis evolved that used a different resource, water, generating oxygen as a by-product.
In those early days, oxygen was poisonous to life. But it built up in the atmosphere until some microbes evolved mechanisms to tolerate it, and eventually hit on ways to use it as an energy source. That was a pretty important discovery too: using oxygen to burn carbohydrates for energy is 18 times as efficient as doing it without oxygen.
Life on Earth became high-powered at this point, setting the scene for the development of complex, multicellular life forms - including plants, which "borrowed" their photosynthetic apparatus from photosynthetic bacteria called cyanobacteria. Today, directly or indirectly, photosynthesis produces virtually all of the energy used by life on Earth.
As well as providing an efficient means to burn fuel, oxygen made by photosynthesis helps protect life. Earth is under constant bombardment from lethal UV radiation streaming out from the sun. A by-product of our oxygenated atmosphere is a layer of ozone extending 20 to 60 kilometres above Earth's surface, which filters out most of the harmful UV. This protective umbrella allowed life to escape from the sanctuary of the ocean and colonise dry land.
“It has altered the atmosphere and cocooned Earth in a protective shield”
Now, virtually every biochemical process on the planet is ultimately dependent on an input of solar energy. Take a deep breath and thank those primordial oxygen-hating microbes for their biochemical inventiveness.
Alison George. Back to top
6. Sex
Birds do it, bees do it - for the vast majority of species, sexual reproduction is the only option. And it is responsible for some of the most impressive biological spectacles on the planet, from mass spawnings of coral so vast that they are visible from space, to elaborate sexual displays such as the dance of the bower bird, the antlers of a stag and - according to some biologists - poetry, music and art. Sex may even be responsible for keeping life itself going: species that give it up almost always go extinct within a few hundred generations.
Important as sex is, however, biologists are still arguing over how it evolved - and why it hasn't un-evolved. That's because, on the face of it, sex looks like a losing strategy.
Evolution ought to favour asexual reproduction for two reasons. First, in the battle for resources, asexual species should be able to outcompete sexual ones hands down. And secondly, because sperm and eggs contain only half of each parent's set of genes, an organism that uses sexual reproduction only gets 50 per cent of its genes into the next generation. Asexuals are guaranteed to pass on 100 per cent.
Clearly, though, there is something wrong with this line of reasoning. It's true that many species, including insects, lizards and plants, do fine without sex, at least for a while. But they are vastly outnumbered by sexual ones.
The enduring success of sex is usually put down to the fact that it shuffles the genetic pack, introducing variation and allowing harmful mutations to be purged (mutations are what eventually snuffs out most asexual species). Variation is important because it allows life to respond to changing environments, including interactions with predators, prey and - particularly - parasites. Reproducing asexually is sometimes compared to buying 100 tickets in a raffle, all with the same number. Far better to have only 50 tickets, each with a different number.
However useful sex may be now that we've got it, that doesn't tell us anything about how it got started. It could have been something as mundane as DNA repair. Single-celled, asexual organisms may have developed the habit of periodically doubling up their genetic material, then halving it again. This would have allowed them to repair any DNA damage by switching in the spare set. A similar exchange of DNA still happens during the production of eggs and sperm.
Parasites are also in the frame. Parasitic lengths of DNA known as transposons reproduce by inserting copies of themselves into the cell's normal genetic material. Imagine a transposon within a single-celled organism acquiring a mutation that happens to cause its host cell to periodically fuse with other cells before dividing again. The transposon for this primitive form of sex would be able to spread horizontally between many different cells. Once it arose in a population, parasitic sex would catch on pretty quickly.
Clare Wilson. Back to top
7. Death
Could evolution have brought the Grim Reaper into being? Yes, indeed. Not in all his guises, of course - living things have always died because of mishaps such as starvation or injury. But there's another sort of death in which cells - and perhaps, controversially, even whole organisms - choose annihilation because of the benefits it brings to some greater whole. In other words, death is an evolutionary strategy.
This is most obvious in the many varieties of programmed cell death or apoptosis, a self-destruct mechanism found in every multicellular organism. Your hand has five fingers because the cells that used to live between them died when you were an embryo. Embryos as tiny as 8 to 16 cells - just 3 or 4 cell divisions after the fertilised egg - depend on cell death: block apoptosis and development goes awry. Were it not for death, we would not even be born.
Even as adults we could not live without death. Without apoptosis we would all be overrun by cancer. Your cells are constantly racking up mutations that threaten to make your tightly controlled cell division run amok. But surveillance systems - such as the one involving the p53 protein, called the "guardian of the genome" (New Scientist, 18 December 2004, p 38) - detect almost all such errors and direct the affected cells to commit suicide.
Programmed cell death plays a central role in everyday life too. It ensures a constant turnover of cells in the gut lining and generates our skin's protective outer layer of dead cells. When the immune system has finished wiping out an infection, the now-redundant white blood cells commit suicide in an orderly fashion to allow the inflammation to wind down. And plants use cell death as part of a scorched-earth defence against pathogens, walling off the infected area and then killing off all the cells within.
It is easy to see how an organism can benefit from sacrificing a few cells. But evolution may also have had a hand in shaping the death of whole organisms. The cells of all higher organisms begin to age, or senesce, after just a few dozen cell divisions, ultimately leading to the death of the organism itself. In part that is one more protection against uncontrolled growth. But one controversial theory suggests this is part of an inbuilt genetic ageing program that sets an upper limit on all our lifespans (New Scientist, 19 April 2004, p 26).
Most evolutionary biologists reject the idea of an innate "death program". After all, they point out, animals die of old age in many different ways, not by one single route as apoptotic cells do. Instead, they view senescence as a sort of evolutionary junkyard: natural selection has little reason to get rid of flaws that appear late in life, since few individuals are lucky enough to make it to old age. But now that people routinely survive well past reproductive age, we suffer the invention evolution never meant us to find: death by old age.
Bob Holmes. Back to top
8. Parasitism
The name is synonymous with stealing, cheating and stealthy evil. But the age-old battle between parasites and their hosts is one of the most powerful driving forces in evolution. Without its plunderers and freeloaders, life would simply not be the same.
From viruses to tapeworms, barnacles to birds, parasites are among the most successful organisms on the planet, taking merciless advantage of every known creature. Take the tapeworm. This streamlined parasite is little more than gonads and a head full of hooks, having dispensed with a gut in favour of bathing in the nutrient-rich depths of its host's digestive system. In its average 18-year lifespan, a human tapeworm can generate 10 billion eggs.
Many parasites, such as the small liver fluke, have also mastered the art of manipulating their host's behaviour. Ants whose brains are infected with a juvenile fluke feel compelled to climb to the tops of grass blades, where they are more likely to be eaten by the fluke's ultimate host, a sheep.
"They are really disgusting, but man, are they good at what they do," says Daniel Simberloff, an ecologist at the University of Tennessee and translator of the popular French text The Art of Being a Parasite. "Evolution is in large part probably driven by parasites. It is the main hypothesis for the continuation of sexual reproduction. How much more important can you get?"
The parasites that have had arguably the biggest effect on evolution are the smallest. Bacteria, protozoans and viruses can shape the evolution of their hosts because only the hardiest will survive infection. And humans are no exception: the genes for several inherited conditions protect against infectious disease when inherited in a single dose. For example, one copy of the gene for sickle cell anaemia protects against malaria. And it is still happening today. HIV and TB, for instance, are driving evolutionary change in parts of our genome, such as the immune-system genes (New Scientist, 22 November 2003, p 44).
Hosts can influence the evolution of their parasites too. For example, diseases which require human-to-human contact for transmission often evolve to be less deadly, ensuring a person will at least live long enough to pass it on.
Parasites can also drive the evolution at a more basic level. Parasitic lengths of DNA called transposons, which can cut and paste themselves all over the genome, can be transformed into new genes or encourage the mutation and shuffling of DNA that fuels genetic variation. They have even been implicated in the origins of sex, as they may have driven selection for cell fusion and gamete formation (see opposite).
Anna Gosline. Back to top
9. Superorganism
LARGE numbers of individuals living together in harmony, achieving a better life by dividing their workload and sharing the fruits of their labours. We call this blissful state utopia, and have been striving to achieve it for at least as long as recorded history. Alas, our efforts so far have been in vain. Evolution, however, has made a rather better job of it.
Take the Portuguese man-of-war. It may look like just another jellyfish blob floating on the high seas, but zoom in with a microscope and you see that what seemed like one tentacled individual is in fact a colony of single-celled organisms. These "siphanophores" have got division of labour down to a fine art. Some are specialised for locomotion, some for feeding, some for distributing nutrients.
This communal existence brings major advantages. It allows the constituent organisms, which would otherwise be rooted to the sea floor, to swim free. And together they are better able to defend themselves against predators, cope with environmental stress, and colonise new territory. Portuguese man-of-war jellyfish are truly superorganisms.
With benefits like these on offer, it should come as no surprise that colonial living has evolved many times. Except that it does come with one big drawback, as the case of the slime bacteria, or myxobacteria, illustrates. These microbes are perhaps the simplest colonial organisms. Under normal circumstances individual bacteria glide along on lonely slime trails. Only when certain amino acids are lacking in their environment do individuals start to aggregate. The resulting superorganism consists of a stalk topped by a fruiting body containing spores. But since only the bacteria forming the spores will get the chance of dispersal and a new life, why should the others play along? How this kind of cooperation evolved, and how cheats are prevented from taking advantage of it remains unclear for some types of colonial life.
But in one group of animals, the colonial insects, we do know what the trick is - and it's an ingenious one. Females develop from fertilised eggs, while males develop from unfertilised ones. This way of determining sex, called haplodiploidy, ensures that sisters are more closely related to each other than to their own offspring. And this means that the best chance they can give their own genes of surviving is to look after each other rather than lay eggs of their own. This is what provides the stability at the heart of the beehive and termite mound, and in many other insect colonies where haplodiploidy has evolved at least a dozen times.
True sociality, or eusociality as it is technically known, is found in all ants and termites, in the most highly organised bees and wasps, and in some other species, not all of which employ haplodiploidy. And although these mini societies need careful policing to keep cheats at bay, this is probably the closest thing on Earth to utopia.
Kate Douglas. Back to top
10. Symbiosis
Crocodiles with gleaming gums, coral reefs, orchids, fish with glow-in-the-dark lures, ants that farm, new directions for evolution. All that from swapping food - for cleaning services, for transport, for sunscreen, for shelter, and of course for other food.
Symbiosis has many definitions, but we'll take it to mean two species engaging in physically intimate, mutually beneficial dependency, almost invariably involving food. Symbiosis has triggered seismic shifts in evolution, and evolution in turn continually spawns new symbiotic relationships.
Perhaps the most pivotal couplings were the ones that turbocharged complex, or eukaryotic, cells. Eukaryotes use specialised organelles such as mitochondria and chloroplasts to extract energy from food or sunlight. These organelles were originally simpler, prokaryotic cells that the eukaryotes engulfed in an eternal symbiotic embrace. Without them life's key developments, such as increasing complexity and multicellular plants and animals, would not have happened. "There are only two things that matter in this world: respiration and photosynthesis. Eukaryotes didn't figure out either by themselves, they borrowed them from prokaryotes through symbiosis," says Geoff McFadden of the University of Melbourne, Australia.
Symbiosis has popped up so frequently during evolution that it is safe to say it's the rule, not the exception. Angler fish in the deep ocean host bioluminescent bacteria in appendages that dangle over their mouths. Smaller fish lured by the light are easy prey. At the ocean surface, coral polyps provide homes for photosynthetic algae, and swap inorganic waste products for organic carbon compounds - one reason why nutrient-poor tropical waters can support so much life. The algae also produce a chemical that absorbs ultraviolet light and protects the coral.
More than 90 per cent of plant species are thought to engage in symbiotic couplings. Orchid seeds are little more than dust, containing next to no nutrients. To germinate and grow, they digest a fungus that infects the seed. "Birds and animals and insects that are adapted to pollination and seed disposal, these are some of the greatest symbioses. Without them we wouldn't have most of our flowering plants," says Ursula Munro, an ecologist at the University of Technology in Sydney, Australia.
“Without symbiosis we wouldn't have most of our flowering plants”
Plovers pick leeches from crocodiles' teeth, offering dental hygiene in return for food. Leafcutter ants use chopped-up leaves as a fertiliser for the fungus they grow in underground chambers. The ants cannot digest the leaves but the fungus that feeds on them produces a tasty meal of sugars and starch while breaking down the toxins in the leaves. And there is not an animal out there, including us, that can survive without the bacteria that live in its gut, digesting food and producing vitamins.
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