Changes in the cultural environment
Both large and small changes in the physical environment can bring about evolutionary change. But even in the absence of such changes, cultural selective pressures that have acted on our species have had a large effect on our evolution and will continue to do so. To understand these pressures, we must put them into the context of hominid history. As we do so, we will see that cultural change has been a strong driving force of human evolution, and has also affected many other species with which we are associated.
About 6 million years ago, in Africa, our evolutionary lineage separated from the lineage that led to chimpanzees and bonobos. The recent discovery in Kenya of chimpanzee teeth that are half a million years old (McBrearty and Jablonski, 2005) shows that the chimpanzee lineage has remained distinct for most of that time from our own lineage, though the process of actual separation of the gene pools may have been a complicated one (Patterson et al., 2006). There is much evidence that our remote ancestors were morphologically closer to chimpanzees and bonobos than to modern humankind ourselves. The early hominid Ardipithecus ramidus, living in East Africa 4.4 million years ago, had skeletal features and a brain size resembling those of chimpanzees (White et al., 1994). Its skeleton differed from those of chimpanzees in only two crucial respects: a slightly more anterior position of the foramen magnum, which is the opening at the bottom of the skull through which the spinal cord passes, and molars with flat crowns like those of modern humans rather than the highly cusped molars of chimpanzees. If we could resurrect an A. ramidus it would probably look very much like a chimpanzee to us - though there is no doubt that A. ramidus and present-day chimpanzees would not recognize each other as members of the same species.
Evolutionary changes in the hominid line include a gradual movement in the direction of upright posture. The changes required a number of coordinated alterations in all parts of the skeleton but in the skull and the pelvis in particular. Perhaps the most striking feature of this movement towards upright posture is how gradual it has been. We can trace the change in posture through the gradual anterior movement of the skull's foramen magnum that can be seen to have taken place from the oldest hominid fossils down to the most recent.
A second morphological change is of great interest. Hominid brains have undergone a substantial increase in size, with the result that modern human brains have more than three times the volume of a chimpanzee brain. Most of these increases have taken place during the last 2.5 million years of our history. The increases took place not only in our own immediate lineage but also in at least one other extinct lineage that branched off about a million years ago - the lineage of Europe and the Middle East that included the pre-Neanderthals and Neanderthals. It is worth emphasizing, however, that this overall evolutionary 'trend' may have counterexamples, in particular the apparent substantial reduction in both brain and body size in the H.Jioresiensis lineage. The remarkable abilities of the hobbits make it clear that brain size is not the only determiner of hominid success.
Our ability to manipulate objects and thus alter our environment has also undergone changes during the same period. A number of changes in the structure of hominid hands, such as the increase in brain size beginning at least 2.5 million years ago, have made them more flexible and sensitive. And our ability to communicate, too, has had a long evolutionary history, reflected in both physical and behavioural characteristics. The Neanderthals had a voice box indistinguishable from our own, suggesting that they were capable of speech (Arensburg et al., 1989). The ability of human children to learn a complex language quickly (and their enthusiasm for doing so) has only limited counterparts in other primates. Although some chimpanzees, bonobos and gorillas have shown remarkable understanding of human spoken language, their ability to produce language and to teach language to others is severely limited (Tagliatela et al., 2003).
Many of the changes in the hominid lineage have taken place since the beginning of the current series of glaciations 2.5 million years ago. There has been an accelerating increase in the number of animal and plant extinctions worldwide during this period. These include extinctions in our own lineage, such as those of H. habilis, H. erectus and H. ergaster, and more recently the Neanderthals and H.floresiensis. These extinctions have been accompanied by a rapid rate of evolution in our own lineage.
What are the cultural pressures that have contributed to this rapid evolution? I have argued elsewhere (Wills, 1993) that a feedback loop involving our brains, our bodies, our genes, and our rapidly changing cultural environment has been an important contributor to morphological and behavioural changes. Feedback loops are common in evolution and have led to many extreme results of sexual selection in animals and of interactions with pollinators among flowering plants. A 'runaway brain' feedback can explain why rapid changes have taken place in the hominid lineage.
The entire human and chimpanzee genomes are now available for comparison, opening up an astounding new world of possibilities for scientific investigation (Chimpanzee Sequencing and Analysis Consortium, 2005). Overall comparisons of the sequences show that some 10 million genetic changes separate us from chimpanzees. We have hardly begun to understand which of these changes have played the most essential role in our evolution. Even at such early stages in these genome-wide investigations, however, we can measure the relative rate of change of different classes of genes as they have diverged in the two lineages leading to humans and chimpanzees. It is now possible to examine the evolution of genes that are involved in brain function in the hominid lineage and to compare these changes with the evolution of the equivalent (homologous) genes in other primates. The first such intergenomic comparisons have now been made between genes that are known to be involved in brain growth and metabolism and genes that affect development and metabolic processes in other tissues of the body. Two types of information have emerged, both of which demonstrate the rapid evolution of the hominid lineage.
First, the genes that are expressed in brain tissue have undergone more regulatory change in the human lineage than they have in other primate lineages. Gene regulation determines whether and when a particular gene is expressed in a particular tissue. Such regulation, which can involve many different interactions between regulatory proteins and stretches of DNA, has a strong influence on how we develop from embryo to adult. As we begin to understand some of these regulatory mechanisms, it is becoming clear that they have played a key role in many evolutionary changes, including major changes in morphology and behaviour. We can examine the rate of evolution of these regulatory changes by comparing the ways in which members of this class of genes are expressed in the brains of ourselves and of our close relatives (Enard et al., 2002). One pattern that often emerges is that a given gene may be expressed at the same level (say high or low) in both chimpanzees and rhesus monkeys, but at a different level (say intermediate) in humans. Numerous genes show similar patterns, indicating that their regulation has undergone significantly more alterations in our lineage than in those of other primates. Unlike the genes involved in brain function, regulatory changes have not occurred preferentially in the hominid lineage in genes expressed in the blood and liver.
Second, genes that are implicated in brain function have undergone more meaningful changes in the human lineage than in other lineages. Genes that code for proteins undergo two types of changes: non-synonymous changes that alter the proteins that the genes code for, possibly changing their function, and synonymous changes that change the genes but have no effect on the proteins. When genes that are involved in brain function are compared among different mammalian lineages, significantly more potentially functional changes have occurred in the hominid lineage than in the other lineages (Clark et al., 2003). This finding shows clearly that in the hominid lineage strong natural selection has changed genes involved in brain function more rapidly than the changes that have taken place in other lineages.
Specific changes in genes that are involved in brain function can now be followed in detail. Evidence is accumulating that six microcephalin genes are involved in the proliferation of neuroblasts during early brain development. One of these genes, MCPH1, has been found to carry a specific haplotype at high frequency throughout human populations (Evans et al., 2005), and it has reached highest frequency in Asia (Fig. 3.1). The haplotype has undergone some further mutations and recombinations since it first arose about 37,000 years ago, and these show strong linkage disequilibrium. Other alleles at this locus do not show such a pattern of disequilibrium. Because disequilibrium breaks down with time, it is clear that this recent haplotype has spread as a result of strong natural selection.
Another gene also associated with microcephaly, abnormal spindle-like microcephaly-associated (ASPM), shows even more recent evidence of extremely strong selection (Mekel-Bobrov et al., 2005). An allele found chiefly in Europe and the Middle East, but at much lower frequencies in Asia (Fig. 3.2), appears to have arisen as recently as 5800 years ago. The spread of this allele has been so rapid that it must confer a selective advantage of several percent on its carriers.
Although both these alleles carry non-synonymous base changes, the allelic differences that are being selected may not be these changes but may be in linked regulatory regions. Further, a direct effect of these alleles on brain function has not been demonstrated. Nonetheless, the geographic patterns seen in these alleles indicate that natural selection is continuing to act powerfully on our species.
Hawks and coworkers (Hawks et al. 2007) have recently shown that the pattern of strong recent selection detectable by linkage disequilibrium extends to more than 2,000 regions of the human genome. They calculate that over the last 40,000 years our species has evolved at a rate 100 times as fast as our previous evolution. Such a rapid rate may require that many of the newly selected genes are maintained by frequency-dependent selection, to reduce the selective burden on our population.
Continue reading here: Ongoing human evolution
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