A brief history of the brain
by David Robson, New Scientist Magazine issue 2831, September 26, 2011.
<img title="Intelligent origins (Image: Burn Everything/Agency Rush)” src=”http://www.newscientist.com/data/images/ns/cms/mg21128311.800/mg21128311.800-1_300.jpg” alt=”Intelligent origins (Image: Burn Everything/Agency Rush)” />Intelligent origins (Image: Burn Everything/Agency Rush)
New Scientist tracks the evolution of our brain from its origin in ancient seas to its dramatic expansion in one ape – and asks why it is now shrinking
IT IS 30,000 years ago. A man enters a narrow cave in what is now the south of France. By the flickering light of a tallow lamp, he eases his way through to the furthest chamber. On one of the stone overhangs, he sketches in charcoal a picture of the head of a bison looming above a woman’s naked body.
In 1933, Pablo Picasso creates a strikingly similar image, called Minotaur Assaulting Girl.
That two artists, separated by 30 millennia, should produce such similar work seems astonishing. But perhaps we shouldn’t be too surprised. Anatomically at least, our brains differ little from those of the people who painted the walls of the Chauvet cave all those years ago. Their art, part of the “creative explosion” of that time, is further evidence that they had brains just like ours.
How did we acquire our beautiful brains? How did the savage struggle for survival produce such an extraordinary object? This is a difficult question to answer, not least because brains do not fossilise. Thanks to the latest technologies, though, we can now trace the brain’s evolution in unprecedented detail, from a time before the very first nerve cells right up to the age of cave art and cubism.
The story of the brain begins in the ancient oceans, long before the first animals appeared. The single-celled organisms that swam or crawled in them may not have had brains, but they did have sophisticated ways of sensing and responding to their environment. “These mechanisms are maintained right through to the evolution of mammals,” says Seth Grant at the Wellcome Trust Sanger Institute in Cambridge, UK. “That’s a very deep ancestry.”
The evolution of multicellular animals depended on cells being able to sense and respond to other cells – to work together. Sponges, for example, filter food from the water they pump through the channels in their bodies. They can slowly inflate and constrict these channels to expel any sediment and prevent them clogging up. These movements are triggered when cells detect chemical messengers like glutamate or GABA, pumped out by other cells in the sponge. These chemicals play a similar role in our brains today (Journal of Experimental Biology, vol 213, p 2310).
Releasing chemicals into the water is a very slow way of communicating with distant cells – it can take a good few minutes for a demosponge to inflate and close its channels. Glass sponges have a faster way: they shoot an electrical pulse across their body that makes all the flagellae that pump water through their bodies stop within a matter of seconds (Nature, vol 387, p 29).
This is possible because all living cells generate an electrical potential across their membranes by pumping out ions. Opening up channels that let ions flow freely across the membrane produces sudden changes in this potential. If nearby ion channels also open up in response, a kind of Mexican wave can travel along a cell’s surface at speeds of several metres a second. Since the cells in glass sponges are fused together, these impulses can travel across their entire bodies.
Recent studies have shown that many of the components needed to transmit electrical signals, and to release and detect chemical signals, are found in single-celled organisms known as choanoflagellates. That is significant because ancient choanoflagellates are thought to have given rise to animals around 850 million years ago.
So almost from the start, the cells within early animals had the potential to communicate with each other using electrical pulses and chemical signals. From there, it was not a big leap for some cells to become specialised for carrying messages.
These nerve cells evolved long, wire-like extensions – axons – for carrying electrical signals over long distances. They still pass signals on to other cells by releasing chemicals such as glutamate, but they do so where they meet them, at synapses. That means the chemicals only have to diffuse across a tiny gap, greatly speeding things up. And so, very early on, the nervous system was born.
The first neurons were probably connected in a diffuse network across the body (see diagram). This kind of structure, known as a nerve net, can still be seen in the quivering bodies of jellyfish and sea anemones.
But in other animals, groups of neurons began to appear – a central nervous system. This allowed information to be processed rather than merely relayed, enabling animals to move and respond to the environment in ever more sophisticated ways. The most specialised groups of neurons – the first brain-like structure – developed near the mouth and primitive eyes.
Our view of this momentous event is hazy. According to many biologists, it happened in a worm-like creature known as the urbilaterian (see diagram), the ancestor of most living animals including vertebrates, molluscs and insects. Strangely, though, some of its descendants, such as the acorn worm, lack this neuronal hub.
It is possible the urbilaterian never had a brain, and that it later evolved many times independently. Or it could be that the ancestors of the acorn worm had a primitive brain and lost it – which suggests the costs of building brains sometimes outweigh the benefits.
Either way, a central, brain-like structure was present in the ancestors of the vertebrates. These primitive, fish-like creatures probably resembled the living lancelet, a jawless filter-feeder. The brain of the lancelet barely stands out from the rest of the spinal cord, but specialised regions are apparent: the hindbrain controls its swimming movements, for instance, while the forebrain is involved in vision. “They are to vertebrates what a small country church is to Notre Dame cathedral – the basic architecture is there though they lack a lot of the complexity,” says Linda Holland at the University of California, San Diego.
Some of these fish-like filter feeders took to attaching themselves to rocks. The swimming larvae of sea squirts have a simple brain but once they settle down on a rock it degenerates and is absorbed into the body.
We would not be here, of course, if our ancestors had not kept swimming. And around 500 million years ago, things went wrong when one of them was reproducing, resulting in its entire genome getting duplicated. In fact, this happened not just once but twice.
These accidents paved the way for the evolution of more complex brains by providing plenty of spare genes that could evolve in different directions and take on new roles. “It’s like the time your parents bought you the biggest Lego kit – with loads of different components to use in different combinations,” says Grant. Among many other things, it enabled different brain regions to express different types of neurotransmitter, which in turn allowed more innovative behaviours to emerge.
As early fish struggled to find food and mates, and dodge predators, many of the core structures still found in our brains evolved: the optic tectum, involved in tracking moving objects with the eyes; the amygdala, which helps us to respond to fearful situations; parts of the limbic system, which gives us our feelings of reward and helps to lay down memories; and the basal ganglia, which control patterns of movements (see diagram).
By 360 million years ago, our ancestors had colonised the land, eventually giving rise to the first mammals about 200 million years ago. These creatures already had a small neocortex – extra layers of neural tissue on the surface of the brain responsible for the complexity and flexibility of mammalian behaviour. How and when did this crucial region evolve? That remains a mystery. Living amphibians and reptiles do not have a direct equivalent, and since their brains do not fill their entire skull cavity, fossils tell us little about the brains of our amphibian and reptilian ancestors.
What is clear is that the brain size of mammals increased relative to their bodies as they struggled to contend with the dinosaurs. By this point, the brain filled the skull, leaving impressions that provide tell-tale signs of the changes leading to this neural expansion.
Timothy Rowe at the University of Texas at Austin recently used CT scans to look at the brain cavities of fossils of two early mammal-like animals, Morganucodon andHadrocodium, both tiny, shrew-like creatures that fed on insects. This kind of study has only recently become feasible. “You could hold these fossils in your hands and know that they have answers about the evolution of the brain, but there was no way to get inside them non-destructively,” he says. “It’s only now that we can get inside their heads.”
Rowe’s scans revealed that the first big increases in size were in the olfactory bulb, suggesting mammals came to rely heavily on their noses to sniff out food. There were also big increases in the regions of the neocortex that map tactile sensations – probably the ruffling of hair in particular – which suggests the sense of touch was vital too (Science, vol 332, p 955). The findings fit in beautifully with the widely held idea that early mammals were nocturnal, hiding during the day and scurrying around in the undergrowth at night when there were fewer hungry dinosaurs running around.
After the dinosaurs were wiped out, about 65 million years ago, some of the mammals that survived took to the trees – the ancestors of the primates. Good eyesight helped them chase insects around trees, which led to an expansion of the visual part of the neocortex. The biggest mental challenge, however, may have been keeping track of their social lives.
If modern primates are anything to go by, their ancestors likely lived in groups. Mastering the social niceties of group living requires a lot of brain power. Robin Dunbar at the University of Oxford thinks this might explain the enormous expansion of the frontal regions of the primate neocortex, particularly in the apes. “You need more computing power to handle those relationships,” he says. Dunbar has shown there is a strong relationship between the size of primate groups, the frequency of their interactions with one another and the size of the frontal neocortex in various species.
Besides increasing in size, these frontal regions also became better connected, both within themselves, and to other parts of the brain that deal with sensory input and motor control. Such changes can even be seen in the individual neurons within these regions, which have evolved more input and output points.
All of which equipped the later primates with an extraordinary ability to integrate and process the information reaching their bodies, and then control their actions based on this kind of deliberative reasoning. Besides increasing their overall intelligence, this eventually leads to some kind of abstract thought: the more the brain processes incoming information, the more it starts to identify and search for overarching patterns that are a step away from the concrete, physical objects in front of the eyes.
Which brings us neatly to an ape that lived about 14 million years ago in Africa. It was a very smart ape but the brains of most of its descendants – orang-utans, gorillas and chimpanzees – do not appear to have changed greatly compared with the branch of its family that led to us. What made us different?
It used to be thought that moving out of the forests and taking to walking on two legs lead to the expansion of our brains. Fossil discoveries, however, show that millions of years after early hominids became bipedal, they still had small brains.
We can only speculate about why their brains began to grow bigger around 2.5 million years ago, but it is possible that serendipity played a part. In other primates, the “bite” muscle exerts a strong force across the whole of the skull, constraining its growth. In our forebears, this muscle was weakened by a single mutation, perhaps opening the way for the skull to expand. This mutation occurred around the same time as the first hominids with weaker jaws and bigger skulls and brains appeared (Nature, vol 428, p 415).
Once we got smart enough to innovate and adopt smarter lifestyles, a positive feedback effect may have kicked in, leading to further brain expansion. “If you want a big brain, you’ve got to feed it,” points out Todd Preuss of Emory University in Atlanta, Georgia.
He thinks the development of tools to kill and butcher animals around 2 million years ago would have been essential for the expansion of the human brain, since meat is such a rich source of nutrients. A richer diet, in turn, would have opened the door to further brain growth.
Primatologist Richard Wrangham at Harvard University thinks that fire played a similar role by allowing us to get more nutrients from our food. Eating cooked food led to the shrinking of our guts, he suggests. Since gut tissue is expensive to grow and maintain, this loss would have freed up precious resources, again favouring further brain growth.
Mathematical models by Luke Rendell and colleagues at the University of St Andrews in the UK not only back the idea that cultural and genetic evolution can feed off each other, they suggest this can produce extremely strong selection pressures that lead to “runaway” evolution of certain traits. This type of feedback might have played a big role in our language skills. Once early humans started speaking, there would be strong selection for mutations that improved this ability, such as the famous FOXP2 gene, which enables the basal ganglia and the cerebellum to lay down the complex motor memories necessary for complex speech.
The overall picture is one of a virtuous cycle involving our diet, culture, technology, social relationships and genes. It led to the modern human brain coming into existence in Africa by about 200,000 years ago.
Evolution never stops, though. According to one recent study, the visual cortex has grown larger in people who migrated from Africa to northern latitudes, perhaps to help make up for the dimmer light up there (Biology Letters, DOI: 10.1098/rsbl.2011.0570).
Downhill from here
So why didn’t our brains get ever bigger? It may be because we reached a point at which the advantages of bigger brains started to be outweighed by the dangers of giving birth to children with big heads. Or it might have been a case of diminishing returns.
Our brains are pretty hungry, burning 20 per cent of our food at a rate of about 15 watts, and any further improvements would be increasingly demanding. Simon Laughlin at the University of Cambridge compares the brain to a sports car, which burns ever more fuel the faster it goes.
One way to speed up our brain, for instance, would be to evolve neurons that can fire more times per second. But to support a 10-fold increase in the “clock speed” of our neurons, our brain would need to burn energy at the same rate as Usain Bolt’s legs during a 100-metre sprint. The 10,000-calorie-a-day diet of Olympic swimmer Michael Phelps would pale in comparison.
Not only did the growth in the size of our brains cease around 200,000 years ago, in the past 10,000 to 15,000 years the average size of the human brain compared with our body has shrunk by 3 or 4 per cent. Some see this as no cause for concern. Size, after all, isn’t everything, and it’s perfectly possible that the brain has simply evolved to make better use of less grey and white matter. That would seem to fit with some genetic studies, which suggest that our brain’s wiring is more efficient now than it was in the past.
Others, however, think this shrinkage is a sign of a slight decline in our general mental abilities. David Geary at the University of Missouri-Columbia, for one, believes that once complex societies developed, the less intelligent could survive on the backs of their smarter peers, whereas in the past, they would have died – or at least failed to find a mate.
This decline may well be continuing. Many studies have found that the more intelligent people are, the fewer children they tend to have. More than ever before, intellectual and economic success are not linked with having a bigger family. If it were, says Rendell, “Bill Gates would have 500 children.”
This evolutionary effect would result in a decline of about 0.8 IQ points per generation in the US if you exclude the effects of immigration, a 2010 study concluded (Intelligence, vol 38, p 220). However, nurture matters as well as nature: even if this genetic effect is real, it has been more than compensated for by improved healthcare and education, which led a steady rise in IQ during most of the 20th century.
Crystal-ball gazing is always a risky business, and we have no way of knowing the challenges that humanity will face over the next millennia. But if they change at all, it appears likely that our brains are going keep “devolving” – unless, of course, we step in and take charge.
The feathered apes
Would intelligent dinosaurs rule the world if a meteorite impact had not wiped out their kind?
We cannot answer that question, of course, but there is no doubt that dinosaurs had the potential to evolve into very smart animals. The proof is sitting in a tree near you.
Certain birds, particularly the crow family, have evolved complex behaviours that match the ingenuity of many primates. Tool use, deception, face recognition – you name it, they can do it. Why are some birds so brainy? Stig Walsh at the National Museums Scotland, thinks that foundations were laid in their dinosaur ancestors, which probably climbed around in trees before eventually taking to the air. This behaviour would have favoured the same abilities that evolved in the tree-climbing primates: excellent vision, motor coordination and balance, which came about through the expansion of the brain areas known as the optic tectum and the cerebellum.
To compete with other animals, these tree-climbing dinosaurs might have also begun to evolve new foraging strategies that needed more brain power, leading to the growth of the forebrain. There are plenty of fossils of dinosaurs, he says, whose brains already possess some of these enlarged structures.
So the ancestors of birds had relatively big brains compared with their body size, and their brains grew proportionately even bigger once they took to the air and evolved even more advanced behaviours. These abilities might have enabled them to survive the mass extinction that killed the other dinosaurs, Walsh says, since their ingenuity would have helped them to find new ways of foraging for food in the wake of the catastrophe.
Bird brains are structured in a very different way to mammalian ones. The mammalian lineage developed new outer layers, known as the neocortex, which birds lack. Despite this, it is likely that the enlarged frontal cortex of the mammals, and the enlarged forebrain of the birds, perform similar functions. “There’s been a convergence, along different routes,” says Walsh.
How smart could birds get? For all the tool-making talents of crows, a beak is clearly not as good for manipulating objects as the hands of primates. That may limit the development of bird brains, though some have speculated that the wings of ground-living birds could yet re-evolve grasping forelimbs.
David Robson is a features editor at New Scientist