Grey matter blues: why has the human brain been shrinking?

There is a disturbing fact about our species that the public don't appear to know, and few specialists seem to want to discuss: over recent millennia, the human brain has been shrinking. There have been plenty of non-scientific warnings about the alleged deleterious effects on IQ of first television and more recently smartphones and tablets, but palaeontological evidences proves that over some tens of thousands of years, the Homo sapiens brain has shrunk somewhere between ten and seventeen percent.

There are usually two key indicators said to provide an accurate measure of smartness: encephalisation quotient and absolute brain size. Encephalisation quotient or EQ is simply the ratio of the mass of the brain to the mass of the body. Overall size is seen as critical due to the number of neural connections required for complex thought processes; you can only squeeze so many neurons into any given volume. Having said that, there is some considerably flexibility around this, thanks to variation in neuron density. The reason that some birds, especially the crow and parrot families are highly intelligent despite the small absolute size of their brains is due to their higher neural density compared to mammals.

Analysis of data from the examination of thousands of human fossil remains suggests that our species reached a peak in EQ around 70,000 years ago, followed by a gradual decline. The reduction in brain size appears to be due to a loss of the archetypal grey matter itself, rather than the white matter that provides support to the neural architecture. However, one key issue is lack of agreement as to a definitive start date for this decline, with 20,000 to 40,000 years ago being the most commonly cited origin. With such basic points remaining unsettled, it's perhaps not surprising that there is a plethora of opinions as to the cause. Here are some of the more popular hypotheses for the decline in human brain size:

1. Change to body size

The first and perhaps most obvious - but easily refuted idea - is that human body size has been steadily declining and so cranial capacity has kept in step with this. While it is true that archaic sapiens may have had a higher mass and even stature than modern humans, the reduction in brain size is greater than would be expected when compared to the overall shrinkage. The assumption is that the development of material culture, from clothing to weapons, has given humans a less calorie-demanding lifestyle.

This would allow - although not dictate - natural selection to trend towards a smaller body size. This doesn't appear to offer any help for the comparatively greater reduction in brain mass, although we should remember that an overall reduction in body size means a smaller birth canal. This in turn requires a smaller skull at birth; as is well known, the human gestation period is three months' less than for similar-size mammals, but our seemingly premature delivery is necessary for the pelvis to maintain efficient bipedalism.

2. Self-domestication

Another idea is that humanity has become domesticated via the impact of culture upon natural selection. Following the population bottleneck of 70,000 years ago - the cause of which is not yet confirmed, despite attempts to correlate it with the Toba super-volcano - there has been continual growth of the human population.

Just as all our domesticated animal species have brain sizes some 10-15% smaller than their wild cousins and ancestors, so the move to larger groups sizes may have led to a more docile humanity, with associated traits such as a smaller cranial capacity being carried along with it.

There are several issues with this hypothesis, ranging from a lack of data on the size of gatherer-hunter bands to the biological mechanisms involved. As regards the latter, there has been some speculation concerning neoteny, in which a species no longer grows to the final stage of maturity. The idea is that if adults are more aggressive than juveniles but peaceful collaboration can lead to larger groups, mutual aid and longer lifespans, then unintentional selective breeding for the retention of juvenile characteristics, including smaller brains, may cause a shift away from the fully mature but more aggressive individuals.

Research in recent years has suggested our brains may continuing to grow into our early thirties rather than cease growing in our teens, so it's possible there could be some truth to this; it would interesting to seek evidence as to whether the brains of archaic sapiens continued growing longer than ours do.

3. The impact of culture

Taking this a step further, increased population density allows a more rapid development and transmission of new ideas, including those that lead to better health, longer lifespans and so to an increased birth rate. Culture and sophisticated language may have reduced the need for most people to gain a wide range of skills - courtesy of a higher intellectual capability - as tasks could be shared and specialisation take hold. In effect, larger societies provide a safety net for those who would be less able to cope in smaller groups.

If ideas could be handed down, then individuals wouldn't have to continually 'reinvent the wheel' in each generation, allowing survival despite a smaller brain size and decreased level of intelligence. The problem with this scenario is that we have no proof the 10-17% reduction has led to an associated drop in intellect; it may well be that the size of certain lobes, used in specialist thought processes such as formulating complex speech, far outweigh any decline in less critical areas.

4. The expensive big brain

One possibility that has a clear cause-and-effect concerns the energy demands of having larger brains. Although they consume a quarter of our daily calories, the human brain is less than five per cent of our body weight. Therefore, there could be a case for arguing the existence of an evolutionary competition between smaller-brained individuals who can survive on less food with those who use their larger brains to improve food collecting strategies. Unfortunately, there are so many variables that it's difficult to judge whether the former would continually trend against the latter and - considering it clearly occurred - why the larger brain managed to evolve in the first place?

5. The more efficient brain

Although a smaller brain might have fewer neurons than a larger version with similar architecture, it has been suggested that its shorter pathways would lead to more rapid thought processing than in a larger counterpart. In addition, there might be fewer neural pathways, again increasing the efficiency. This 'nimble thinking' approach certainly seems logical, although again it doesn't explain the evolution of larger EQ in archaic sapiens.

This is certainly a subject ripe for much more research. I've often concluded with a statement along the lines that it wouldn't be surprising if some or all these factors were involved, since nature rarely conforms to the nice, neat patterns we would like to lay upon it. There is an even possibility that brain size - like so many other aspects of all animal species - fluctuates around a mean value, so that what goes up may come down again, only to later go up again.

At least one anthropological study on both Afro-Americans and US citizens of European descent proposes that over the past few hundred years there may have been an upward drift towards larger brains. Assuming the research is accurate, one possibility is that the superior nutrition available since the Industrial Revolution is allowing such development, thanks to the comparative ease with which its energy demands can be fulfilled.

It would certainly be interesting to investigate this hypothesis on a global scale, considering the wide differences between the clinically obese nations and those still subject to frequent famine. Whatever the results, they are unlikely to be the simple 'just-so' stories often passed-off as to the public in lieu of accurate but understandable science communication. The answers may be out there somewhere...I'd certainly love to know what's been happening to the most sophisticated object in the known universe!

Ancestral tales: why we prefer fables to fact for human evolution

It seems that barely a month goes by without there being a news article concerning human ancestry. In the eight years since I wrote a post on the apparent dearth of funding in hominin palaeontology there appears to have been some uptake in the amount of research in the field. This is all to the good of course, but what is surprising is that much of the non-specialist journalism - and therefore public opinion - is still riddled with fundamental flaws concerning both our origins and evolution in general.

It also seems that our traditional views of humanity's position in the cosmos is often the source of the errors. It's one thing to make such howlers as the BBC News website did some years' back, in which they claimed chimpanzees were direct human ancestors, but there are a key number of more subtle errors that are repeated time and again. What's interesting is that in order to explain evolution by natural selection, words and phrases have become imbued with incorrect meaning or in some cases, just a slight shift of emphasis. Either way, it seems that evolutionary ideas have been tacked onto existing cultural baggage and in the process, failed to explain the intended theories; personal and socio-political truths have triumphed over objective truth, as Neil deGrasse Tyson might say.

1) As evolutionary biologist Stephen Jay Gould use to constantly point out, the tree of life is like the branches of a bush, not a ladder of linear progression. It's still fairly common to see the phrase 'missing link' applied to our ancestry, among others; I even saw David Attenborough mention it in a tv series about three years' ago. A recent news article described - as if in surprise - that there were at least three species of hominins living in Africa during the past few million years, at the same time and in overlapping regions too. Even college textbooks use it - albeit in quotation marks - among a plethora of other phrases that were once valid, so perhaps it isn't surprising that popular publications continue to use them without qualification.

Evolution isn't a simple, one-way journey through space and time from ancestors to descendants: separate but contemporaneous child species can arise via geographical isolation and then migrate to a common location, all while their parent species continues to exist. An example today would be the lesser black-backed and herring gulls of the Arctic circle, which is either a single, variable species or two clearly distinct species, depending where you look within its range.

It might seem obvious, but species also migrate and then their descendants return to the ancestral homeland; the earliest apes evolved in Africa and then migrated to south-east Asia, some evolving into the ancestors of gibbons and orangutan while others returned to Africa to become the ancestors of gorillas and chimpanzees. One probable culprit of the linear progression model is that some of the examples chosen to teach evolution such as the horse have few branches in their ancestry, giving the false impression of a ladder in which a descendant species always replaces an earlier one.

2) What defines a species is also much misunderstood. The standard description doesn't do any favours in disentangling human evolution; this is where Richard Dawkins' oft-repeated phrase 'the tyranny of the discontinuous mind' comes into play. Examine a range of diagrams for our family tree and you'll find distinct variations, with certain species sometimes being shown as direct ancestors and sometimes as cousins on extinct branches.

If Homo heidelbergensis is the main root stock of modern humans but some of us have small amounts of Neanderthal and/or Denisovan DNA, then do all three qualify as direct ancestors of modern humans? Just where do you draw the line, bearing in mind every generation could breed with both the one before and after? Even with rapid speciation events between long periods of limited variability (A.K.A. punctuated equilibrium) there is no clear cut-off point separating us from them. Yet it's very rare to see Neanderthals labelled as Homo sapiens neanderthalensis and much more common to see them listed as Homo neanderthalensis, implying a wholly separate species.

Are the religious beliefs and easy-to-digest just-so stories blinding us to the complex, muddled background of our origins? Obviously, the word 'race' has profoundly negative connotations these days, with old-school human variation now known to be plain wrong. For example, there's greater genetic variation in the present-day sub-Saharan African population than in the rest of the world combined, thanks to it being the homeland of all hominin species and the out-of-Africa migrations of modern humans occurring relatively recently.

We should also consider that species can be separated by behaviour, not just obvious physical differences. Something as simple as the different pitches of mating calls separate some frog species, with scientific experiments proving that the animals can be fooled by artificially changing the pitch. Also, just because species appear physically similar doesn't necessarily mean an evolutionary close relationship: humans and all other vertebrates are far closer to spiny sea urchins and knobbly sea cucumbers than they are to any land invertebrates such as the insects.

3) Since the Industrial Revolution, societies - at least in the West - have become obsessed with growth, progress and advance. This bias has clearly affected the popular conception that evolution always leads to improvements, along the lines of faster cheetahs to catch more nimble gazelles and 'survival of the fittest'. Books speak of our epoch as the Age of Mammals, when by most important criteria we live in the era of microbes; just think of the oxygen-generating cyanobacteria. Many diagrams of evolutionary trees place humans on the central axis and/or at the pinnacle, as if we were destined to be the best thing that over three billion years of natural selection could achieve. Of course, this is no better than what many religions have said, whereby humans are the end goal of the creator and the planet is ours to exploit and despoil as we like (let's face it, for a large proportion of our existence, modern Homo sapiens was clearly less well adapted to glacial conditions than the Neanderthals).

Above all, these charts give the impression of a clear direction for evolution with mammals as the core animal branch. Popular accounts still describe our distant ancestors, the synapsids, as the 'mammal-like reptiles', even though they evolved from a common ancestor of reptiles, not from reptiles per se. Even if this is purely due to lazy copying from old sources rather than fact-checking, doesn't it belie the main point of the publication? Few general-audience articles admit that all of the earliest dinosaurs were bipedal, presumably because we would like to conflate standing on two legs with more intelligent or 'advanced' (a tricky word to use in a strict evolutionary sense) lineages.

The old ladder of fish-amphibian-reptile/bird-mammal still hangs over us and we seem unwilling to admit to extinct groups (technically called clades) that break our neat patterns. Incidentally, for the past 100 million years or so, about half of all vertebrate species have been teleost fish - so much for the Age of Mammals! No-one would describe the immensely successful but long-extinct trilobites as just being 'pill bug-like marine beetles' or similar, yet when it comes to humans, we have a definite sore spot. There is a deep psychological need to have an obvious series of ever-more sophisticated ancestors paving the way for us.

What many people don't realise is that organisms frequently evolve both physical and behavioural attributes that are subsequently lost and possibly later regained. Some have devolved into far simpler forms, frequently becoming parasites. Viruses are themselves a simplified life form, unable to reproduce without a high-jacked cell doing the work for them; no-one could accuse them of not being highly successful - as we are currently finding out to our cost. We ourselves are highly adaptable generalists, but on a component-by-component level it would appear that only our brains make us as successful as we are. Let's face it, physically we're not up to much: even cephalopods such as squid and octopus have a form of camera eye that is superior to that of all vertebrates.

Even a cursory glance at the natural history of life, using scientific disciplines as disparate as palaeontology and comparative DNA analysis, shows that some lineages proved so successful that their outward physiology has changed very little. Today, there are over thirty species of lancelet that are placed at the base of the chordates and therefore closely related to the ancestors of all vertebrates. They are also extremely similar in appearance to 530-million-year-old fossils of the earliest chordates in the Cambrian period. If evolution were a one-way ticket to progress, why have they not long since been replaced by later, more sophisticated organisms?

4) We appear to conflate success simply with being in existence today, yet our species is a newcomer and barely out of the cradle compared to some old-timers. We recently learned that Neanderthals wove plant fibre to make string and ate a wide variety of seafood. This knowledge brings with it a dwindling uniqueness for modern Homo sapiens. The frequently given explanation of our superiority over our extinct cousins is simply that they aren't around anymore, except as minor components of our genome. But this is a tautology: they are inferior because they are extinct and therefore an evolutionary dead end; yet they became extinct because of their inferiority. Hmmm...there's not much science going on here!

The usual story until recently was that at some point (often centred around 40,000-50,000 years ago) archaic sapiens developed modern human behaviour, principally in the form of imaginative, symbolic thinking. This of course ignores the (admittedly tentative) archaeological evidence of Neanderthal cave-painting, jewelry and ritual, all of which are supposed to be evidence of our direct ancestor's unique Great Leap Forward (yes, it was named after Chairman Mao's plan). Not only did Neanderthals have this symbolic behaviour, they appear to have developed it independently of genetically-modern humans. This is a complete about-turn from the previous position of them being nothing more than poor copyists.

There are alternative hypotheses to the Great Leap Forward, including:

1. Founder of the Comparative Cognition Project and primate researcher Sarah Boysen observed that chimpanzees can create new methods for problem solving and processing information. Therefore, a gradual accumulation of cognitive abilities and behavioural traits over many millennia - and partially inherited from earlier species - may have reached a tipping point. 
2. Some geneticists consider there to have been a sudden paradigm shift caused by a mutation of the FOXP2 gene, leading to sophisticated language and all that it entails.
3. Other researchers consider that once a certain population size and density was achieved, complex interactions between individuals led the way to modern behaviour. 
4. A better diet, principally in the form of larger amounts of cooked meat, led to increased cognition. 

In some ways, all of these are partly speculative and as is often the case we may eventually find that a combination of these plus other factors were involved. This shouldn't stop us from realising how poor the communication of evolutionary theories still is and how many misconceptions exist, with the complex truth obscured by our need to feel special and to tell simple stories that rarely convey the amazing evolution of life on Earth.