Human descent has long been described to involve macroevolutionary changes in the anatomy, physiology and behavior in the primate species. The prominent enlargement of the primate brain from the New and Old World monkeys (e. g. , tarsiers, colobus monkeys, squirrel monkeys,) to the Great Ape species (e. g. , chimpanzee, gorilla, orangutan, humans) has been identified to be the driving force for primate intelligence.
Consequences of larger brains include mate selection, social group organization, competition and offspring caring. In addition, microevolutionary changes such as gross chromosomal rearrangements, point mutations and gene duplications, may also be involved in the shaping of the primate genome, which in turn, also result in the evolution of the primate brain. Two genes have been recently identified to play a major role in brain enlargement in human evolution.
The ASPM (abnormal spindle-like microcephaly associated) gene is responsible for the size of the cerebral cortex, wherein homozygous mutations in this gene result in microcephaly, a congenital condition presenting a underdeveloped or small head, with the result of the parts of the body at normal size (Mochida and Walsh, 2001). Five recessive alleles have been associated to this clinical condition. Homologues of this gene have been identified in other species—the Aspm gene in the mouse, the Aspm gene in the rate, the asp gene in Drosophila.
Sequencing analysis of the ASPM gene has shown the number of nucleotide substitutions that have occurred among different primate species. Synonymous substitutions are nucleotide mutations that result in the same amino acid, while non-synonymous substitutions are base-pair changes that generate a different or new amino acid. Calculation of the ratio of synonymous to non-synonymous substitutions measures the mutation rate of the gene in each species along the evolutionary tree. The analysis of Evans et al. 2004) showed that there was dramatic increase in the rate of mutation in the ASPM gene among higher primates, which are lineages that are more closely related to the humans. Such values indicate that there was positive selection taking place, especially during the human-chimpanzee divergence, at a gradual evolutionary pace. Such observation is consistent with the progressive emergence of the higher primate species, alongside the gradual enlargement of the primate brain during human lineage.
Is has been estimated that such changes started approximately 18 million years ago (mya) and continued on to the human lineage. Positive selection involves a significant excess of non-synonymous nucleotide substitutions at accelerated rates, resulting in a sudden change in the protein product and its function, and consequently the evolution of the brain-specific protein. If positive selection were not present, the protein will continue to exist in a neutral setting, wherein the same protein will be present across the different species in the primate evolutionary tree.
The causal factor of the positive selection of the ASPM gene has still not been identified, however, it has been suggested that population size may have played a role in the positive selection of the gene. The biochemical function of the ASPM gene has not been determined in humans, but based on the homologues in Drosophila, the gene influence mitotic and meiotic spindle structures. The cells of the developing brain go through successive cell divisions, with the mitotic spindles influencing the plane of division to generate two identical nerve cells.
The orientation of the mitotic spindle has been determined to influence the size of the cerebral cortex of the brain, wherein more symmetric cell divisions result in a bigger brain size. The ASPM homologue is also expressed in the mouse, with symmetry also playing a prime role in the final size of the mouse brain. This has not yet been tested in humans. Mutation rates in the ASPM gene were also tested in several human populations derived from the Coriell Human Variation Panel. These included DNA samples from individuals from North America, Andes, Southeast Asia and Middle East.
It was observed there was a significant increase in non-synonymous substitutions during the divergence between chimpanzee and human species, much greater than the number of non-synonymous substitutions within the human species alone. Such observation may be due to changes in the population size from chimpanzee to humans, since the size of the human population has been constant for the last couple of centuries. These results suggested that there is a strong positive selection occurring during human descent from the chimpanzees, while the number of non-synonymous substitutions is a condition of adaptive evolution in the human species.
It has also been estimated that there are 19 non-synonymous substitutions that have taken place since the chimpanzee-human divergence, of which 15 of these non-synonymous substitutions were fixed into the human lineage. A second important gene involved in the control of brain size is the microcephalin gene. Similar to the ASPM gene, mutations in the microcephalin gene results in a loss-of-function in the amino acids, resulting in microcephaly or decrease in brain volume but a maintenance in the rest of the architecture of the brain.
The exact function of the protein product of the microcephalin gene is unknown, but it has been suggested that it plays an important role in the creation of more neural cells. It has also been shown that the microcephalin gene has been a preferred location for positive selection during primate/hominoid/human evolution, hence this gene is thought to control the evolution of the primate brain size. The research study conducted by Evans et al. (2006) has identified several haplotypes or linked genetic markers that have been transmitted horizontally across different human populations.
One of the haplotypes (haplotype D) was determined to have recently arisen 37,000 years ago, suggesting a sudden increase in its frequency in the human population. Such observation is incompatible with genetic drift and instead follows the mode of positive selection to increase its distribution across the human population. The research has described the origin of this new haplotype approximately 1. 1 million years ago (mya) and subsequently distributed copies throughout the rest of the human population.
A model of population subdivision has been suggested, wherein the human-chimpanzee lineage diverged from the common Homo lineage and the two groups were reproductively isolated for approximately 1. 1 million years ago. During this isolation period, haplotype D was distributed and eventually fixed within the microcephalin gene of the non-human lineage and the non-haplotype D allele was fixed in the modern human lineage. The two haplotypes can be differentiation by several nucleotide substitutions of approximately 37,000 years ago.
However, interbreeding has introduced the haplotype D into the modern human lineage. The original haplotype D-bearing lineage has already gone extinct, but the introduced copy in the modern human lineage has subsequently spread out across the rest of the human populations around the world at high numbers. Such investigation into genes of deep origins or genealogies have been determined to be difficult to assess, yet still possible. Such deep origins may be due to admixture of different reproductively isolated populations, which still exist around the world to date.
These are also present in populations that have been isolated for very long periods of time, due to physical barriers. There are others genes that are associated with reproductively isolated populations, including that of the MAPT locus, which has been estimated to diverge approximately 3 million years ago. The MAPT locus has two distinct haplogroups, H1 and H2, with the H2 haplogroup younger, possibly due to a later introduction or introgression into the modern human lineage and consequently a much later distribution across different human populations.
Also, haplotypes in the dystrophin gene shows that different haplotypes are found in different regions or continents of the world (Labuda et al. 2000). These investigations into the origin of moderns human from the original Homo population through the use of the ASPM and microcephalin genes needs comprehensive analysis of nucleotide substitutions as well as correlation with fossil records on the anatomy of the pre-human conditions. It has been estimated that modern humans and Neanderthals existed at the same time 130,000 years ago in the Middle East, consistent with the estimates on the existence or creation of the microcephalin D allele.
In addition, the frequency distribution of haplotype D across the world is exceptionally high in other parts of the world except Africa, and subsequently very low in the sub-Saharan region, suggesting that there must have been some kind of introduction or introgression into the Eurasian region. In addition, the estimate on the separation time between the D and non-D haplotypes in the range of 1. 1 million years ago is consistent with divergence times estimated from mitochondrial DNA sequence differences between modern human and Neanderthals.
The description of two major brain-specific genes associated with brain size evolution in human evolution provides genetic evidence for the introduction of new forms of genes or alleles in modern human populations that have originated from pre-human lineages. These researches also provide support for the theory that the biological evolution of the human species may have been facilitated by the creation of adaptive alleles, which in turn where created by a battery of point mutations.
Such positive selection of “better” alleles provided better adaptation of the human lineages in different regions around the world. Once this has happened, each specific population in a certain region would flourish, and these individuals eventually colonize the region as their own biological domain. Should this be verified as the main mechanism for the evolution of the human brain, other genes in the human genome may be studied to gain a better understanding of human physiology and disease.