The rest, as they say, is history.
It is not yet clear why our ancestors stayed in more or less the same place for 100,000 years and suddenly, in the span of a mere 40,000 years, settled an entirely new continent. Archaeological evidence suggests that at about the time of the 'great human migration' AMH were becoming ‘smarter’- their technology became more sophisticated, social networks became more complex, and their skills and practices more innovative. It has previously been noted that at about this time our ancestors had developed a voracious appetite for seafood, which contains the fatty acids which are essential for proper brain function and development. Fish and shellfish are rich sources of docosahexaenoic acid (DHA) and arachidonic acid (AA), and so seafood at the very least provided a means of support for our preternaturally large brains and may have contributed to the intelligence boom.
The authors of an article recently published in PLOS ONE reported finding beneficial mutations in the FADS gene cluster which appeared and rapidly spread prior to the great human migration 80,000 years ago, adding another dimension to the story. The FADS gene cluster is found on the 11th chromosome and encodes enzymes which are involved in endogenous DHA synthesis. DHA is a long-chain polyunsaturated fatty acid (LC-PUFA), and we do have a limited capacity for synthesizing LC-PUFAs from the medium chain polyunsaturated fatty acids found in plants (MC-PUFAs).
DHA is particularly tricky to make, however, and so we often rely on diet to satisfy the brain’s DHA requirement, but approximately 85,000 years ago- shortly before the great human migration- a game-changer appeared. Researchers analyzed the genomes of 1,092 people who were part of the 1000 Genomes Project, in which 14 different populations are represented. They found that it was at about this time that several FADS gene variants began to spread throughout the population with increased frequency (a phenomenon known as positive selection), variants which are associated with increased efficiency of LC-PUFA synthesis. The authors confirmed positive selection at the FADS gene cluster by obtaining the DNA sequence from an additional 1,043 individuals from the Human Genome Diversity Panel, representing 51 different populations.
Increased efficiency of LC-PUFA synthesis from MC-PUFAs in conjunction with those readily obtained from fish and shellfish meant our ancestors were better able to support their growing and increasingly complex brains. Once the burden of obtaining DHA and AA from diet alone was relieved, modern humans no longer needed to remain in close proximity to water and instead could afford to be more adventurous while still satisfying the brain’s requirement for LC-PUFAs.
For reasons unknown the beneficial FADS alleles have remained firmly entrenched among African populations but have slowly been phased out of European and Asian populations, occurring with much less frequency now than when they first appeared in our ancestors. The authors speculate that as humans began migrating out of Africa and developing more sophisticated technology and infrastructure these variants provided no significant benefit; rather, endogenous LC-PUFA synthesis became too metabolically taxing when LC-PUFAs could be readily obtained from outside sources. It is also likely that beneficial variants of other unrelated genes eventually appeared, altering the conditions required for metabolic homeostasis.
The timing of the appearance of beneficial FADS mutations was certainly fortuitous, and may have helped our ancestors along by enabling them to decrease their reliance on fish and shellfish as their primary sources of dietary DHA and AA. Correspondingly, the increased ability to synthesize endogenous LC-PUFAs from plant-based sources more efficiently kept their brains satisfied and may have altered the 'cognitive capacities' of early humans- in other words, made AMH smarter. These findings, while not sufficient to entirely explain the migration of humans throughout and finally out of Africa, support the hypothesis that the great human migration had a genetic component.
The contribution of the FADS mutations in the context of the great human migration is impossible to analyze at this point, as we do not yet have a clear picture of what prompted the burst of activity among AMH; but it is valuable information which may help clear the shroud of mystery veiling the sudden migration of AMH 80,000 years ago.
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