I had an email discussion with fellow blogger Eilif Heyerdahl recently, and I speculated that we humans have traded muscular strength for brain power, based on the notion that chimpanzees are 3 to 5 times stronger than humans. This is by no means my original idea. Trade-offs are well known in evolution. But the literature on human evolution offers a far more nuanced and complicated picture than this. Underlying this idea is a fundamental question: What genes differ between humans and chimps, and do they explain the physical differences between the two species?
An article in the MIT Technology review discussed a study of chimpanzee and human genomes, looking for evidence of recent positive natural selection among some 14,000 genes held in common between the two species. This can be done by looking for an excess of single base pair mutations that cause amino acid sequence changes in proteins over mutations that do not. The macaque genome was used as a reference. Mutations that cause amino acid changes can change protein function, and the mutations will be eliminated from the population unless they help the organism have more offspring. Some mutations are neutral, neither benefiting nor harming the organism, because they do not change the amino acid sequence. A gene with more amino acid changes (compared to the reference genome) than neutral mutations is considered positively selected, since it has not been eliminated and yet has caused an altered amino acid sequence. In this study the chimpanzee genome shows evidence of natural selection in 233 (out of ca. 14,000) genes, compared with humans showing evidence of natural selection in 154 (out of ca. 14,000) genes. Disappointingly perhaps, only a few of the human positively selected genes have anything to do with brain development. Some of them are even involved in human diseases, suggesting that although they may have benefited humans at first, they may now reflect the poor adaptation of the human genome to our current way of life. The identities of the positively selected genes, some of which are the same in both species, do not suggest any particular pattern that is correlated with the life style and habitat of either organism. The genetic relationships among humans and the great apes are complex. About one third of the genes the two species have in common are completely identical. The rest typically differ by no more than two amino acids. Given that the structures of proteins do not seem to differ much between the two species, some scientists propose that changes in gene regulation are responsible for the differences that we see between the two species. There are differences in the pattern of expression of genes in humans vs chimpanzees, including genes that are expressed in the brain. More may exist. Research continues on non-regulatory genes as well. A nonsense mutation in the gene for a muscle protein prevents that protein from being made, and contributes to a lower facial muscle mass in humans compared with chimpanzees. Another recent study describes a gene, microcephalin, that has evolved rapidly in the last 37,000 years, and that is expressed in the brain. This modification, of course, is far too recent to explain the split between humans and chimps, which occurred millions of years ago. It does suggest that human brain evolution is still happening. An analysis of metabolites present in various tissues, including muscle, indicates that there has been a major increase in the number of these in humans as compared with chimps. This could happen because some genes can code for more than one protein, depending on how their RNA products are processed before being sent to the cytoplasm to direct protein synthesis. Much research remains to be done to investigate the molecular basis of the species differences.
There are many differences between humans and chimps, including muscle strength. Even if we knew the molecular basis for all these, it would be hard to say which of them are the most important.
Another problem with idea of a simple trade-off of brains for brawn is that it compares two existing species, the chimp and the human. Ideally one would like to trace the changes in both lineages from the common ancestor. There are several archaic ape species that existed at about the right time, going back as far as 7 million years or so. One of these is Ardipithecus ramidus, a possible ancestral hominin that lived about 4.4 million years ago. Any of these archaic ape species could represent a common ancestor or a close relative of that common ancestor. Each is considerably different anatomically from both chimpanzees and humans, which is consistent with the genetic data showing evolution in both lineages. We did not evolve from chimpanzees. Instead, both we and the chimps have changed since the last common ancestor. What happened along each path?
There is a small amount of evidence along the chimpanzee lineage, indicating that chimpanzees existed by about 780,000 years ago. Unfortunately the fossil record for chimpanzees is very poor. This might be because fossils tend not to form in forest habitats where chimps live, or it might be due in part to too little effort on the part of paleontologists. For whatever reasons, the fossil record for humans is much better than that for chimps. The Australopithecines, dating from a little over 3 million years to as recently as just under 2 million years, are considered to be on the human line of descent from the common ancestor. (That is why we group them among the hominins). They walked upright habitually, but they had relatively long arms with curved fingers suitable for climbing trees, so were also likely arboreal part of the time. They had brains comparable in size to or a little larger than those of chimpanzees. The early Australopithecines were only about four feet tall. The most recent species of Australopithecines, around 2.5 million years ago, may have made stone tools. By 1.9 million years ago, there was a new species, Homo habilis, that had a larger brain than the Australopithecines and was definitely associated with stone tools. A more sophisticated species, Homo erectus, emerged by 1.2 million years ago in Africa and migrated to Asia and Indonesia. It had a cranial capacity about half of our own. Homo erectus definitely made tools and hunted animals, but also ate a lot of vegetable material, the main food supply of all hominins up to that time.
After Homo erectus, increases in brain size continued in various hominin species. Although some estimates of body size have been made in the past, there are difficulties in correlating this with brain size because few fossils exist which reliably permit the association of limb bones with skulls. The literature is complex on this subject, with a 1996 report indicating a strict correlation of brain and body size, and another suggesting that body size has not changed between the late Australopithecines and the rise of Homo sapiens . The latter study reported that body size in Homo sapiens declined about 10% after the present day brain size was achieved. Brain size increased dramatically between the time of Homo erectus and the rise of Neanderthals and Homo sapiens. We are certain that human brains are about three times the size of chimpanzee brains, and that these brains drive very different behaviors. However, it does not appear to be the case that an increase in brain size can be taken as a measure of changes in intelligence. Horse brains have also tripled in size! More likely it is the details of the development of the brain that account for the intelligence of humans, rather than just size.
I have a friend, a biologist, who tells me that strength can be estimated from bone structure. This might allow us to focus on strength more than mere body size. The finding of more fossils will also likely improve our picture, and possibly change it considerably.
If the picture seems confusing, then I have conveyed my state of mind about the correlation of brain and body size over the course of human evolution. There is no information about the course of chimpanzee evolution during the same period. It is clear that the human brain is bigger relative to body size than that of the chimp, but how this happened or why remains to be decided. More on this in the next post!