January 20, 2010
2 min read
A new approach to probe ancient regions of the genome suggests early human populations were scarce
New genetic findings suggest that early humans living about one million years ago were extremely close to extinction.
The genetic evidence suggests that the effective population—an indicator of genetic diversity—of early human species back then, including hom*o erectus, H. ergaster and archaic H. sapiens, was about 18,500 individuals (it is thought that modern humans evolved from H. erectus), says Lynn Jorde, a human geneticist at the University of Utah in Salt Lake City. That figure translates into a total population of 55,500 individuals, tops.
One might assume that hominin numbers were expanding at that time as fossil evidence shows that members of our hom*o genus were spreading across Africa, Asia and Europe, Jorde says. But the current study by Jorde and his colleagues suggests instead that the population and, thus its genetic diversity, faced a major setback about one million years ago. The finding is detailed in the January 18 issue of Proceedings of the National Academy of Sciences.
To make these estimates, Jorde's group scanned two completely sequenced modern human genomes for a type of mobile element called Alu sequences. Alu sequences are short snippets of DNA that move between regions of the genome, though with such low frequency that their presence in a region suggests it is quite ancient. Because older Alu-containing regions have had time to accumulate more mutations, the team was also able to estimate the age of a region based on its nucleotide diversity. The team then compared the nucleotides in these old regions with the overall diversity in the two genomes to estimate differences in effective population size, and thus genetic diversity between modern and early humans.
"This is an original approach because they show that you can use mobile elements…to flag a region of the genome," says Cédric Feschotte, an evolutionary geneticist at the University of Texas Arlington.
The effective population researchers estimate at about 18,500 reveals that the extent of genetic diversity among hominins living one million years ago was between 1.7 and 2.9 times greater than among humans today. (Other studies have shown that the present-day effective population is around 10,000.) Jorde says the reason the modern effective population is so much smaller than the current number of people (nearly seven billion) is that a population explosion occurred, probably due to the development of agriculture about 10,000 years ago. He does not expect that there would have been such a staggering difference between the effective and actual populations of early humans.
Jorde thinks that the diminished genetic diversity one million years ago suggests human ancestors experienced a catastrophic event at that time as devastating as a purported supervolcano thought to have nearly annihilated humans 70,000 years ago. "We've gone through these cycles where we've had large population size but also where our population has been very, very small," he says.
As someone deeply versed in genetics and genomics, I can confidently affirm the significance of the methodologies and findings presented in the article. The use of Alu sequences to probe ancient regions of the genome is a well-established technique in the field of molecular anthropology and evolutionary genetics.
Alu Sequences:
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Nature of Alu Sequences: Alu elements are short, repetitive sequences of DNA found abundantly in the human genome. They are classified as retrotransposons, which means they can move around the genome via a copy-and-paste mechanism mediated by an enzyme called reverse transcriptase.
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Ancient Markers: The presence of Alu sequences in specific regions of the genome often indicates ancient DNA segments. Since these sequences insert themselves at various points throughout evolutionary history but rarely move afterward, they serve as markers for estimating the age and diversity of genomic regions.
Effective Population Size:
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Definition: Effective population size (Ne) is a concept used in population genetics to describe the size of an idealized population that would show the same amount of genetic diversity as the population under study.
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Significance of 18,500 Estimate: The estimate of an effective population size of about 18,500 for early humans suggests that the genetic diversity among these ancestral populations was relatively limited compared to what might be expected based on fossil evidence.
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Modern vs. Ancient Diversity: Interestingly, the genetic diversity among early hominins, including hom*o erectus and H. ergaster, was estimated to be between 1.7 and 2.9 times greater than modern humans. This challenges the intuitive assumption that ancient populations would have been more genetically diverse due to broader geographic distributions.
Implications and Hypotheses:
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Population Bottleneck: The reduced effective population size around one million years ago indicates a potential population bottleneck—a sharp reduction in population size leading to reduced genetic diversity. This bottleneck could have resulted from catastrophic events, much like the supervolcano event hypothesized to have occurred 70,000 years ago.
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Agriculture and Modern Populations: The disparity between the effective population size and the actual population of modern humans (nearly seven billion) can be attributed to recent population explosions linked to agricultural development approximately 10,000 years ago.
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Cyclic Population Dynamics: Jorde's observations point to cyclic patterns in human population dynamics, oscillating between phases of significant population sizes and drastic reductions. Such patterns have profound implications for understanding human evolutionary history and susceptibility to environmental factors.
In summary, the utilization of Alu sequences and the estimation of effective population sizes offer invaluable insights into the genetic history of early humans. These findings challenge conventional wisdom and underscore the dynamic nature of human population dynamics over evolutionary timescales.
