Modern humans are believed to have evolved from an archaic lineage that yielded the Denisovans and Neanderthals, but little is known about the evolutional relationships that connect the two groups.
Now, a team of scientists from the University of Utah has developed a new scientific tool to help uncover the mystery. The new technique allows evolutionary researchers to analyze DNA sequence data and uncover historical secrets about archaic human populations.
On using the tool, the University of Utah team was surprised to learn of a contradicting evolutionary story to that recited today about modern humans, Denisovans, and Neanderthals.
What's the new story?
The discovery discloses that the lineage of the Neanderthals and the Denisovans was on the verge of extinction after separating from humans.But total split-up between the two groups did not materialize until 300 generations later (about 744,000 years ago).
What ensued next was a global upsurge of Neanderthal populations, where the total number of individuals grew to tens of thousands.By then, individuals living in fragmented, isolated populations across Eurasia.
This discovery contradicts the conventional hypothesis that estimates the entire Neanderthal population at 1000.Even so, the accuracy of this estimation had already been doubted in a 2015 study, in which researchers claimed that the estimate was an underrepresentation of the total number individuals in all regionally isolated Neanderthal populations.
With limited samples of fossil fragments, the new study relied on genetics and statistics to puzzle out the obscurity.
The research team devised and improved the legofit statistical method, which accounts for multiple populations in a gene pool, and used it to estimate the percentage of Neanderthal genes appearing in modern Eurasian populations, the time at which the two archaic groups separated from each other, and the magnitude of their populations.
Typically, the human genome contains around 3.5 billion nucleotide sites, and genes at some of these sites can mutate and form some genetic stamp on a family's DNA when passed down from a parent to their kids, the next generation, and so on.
Scientists in the latest study used these mutations to look for the missing pieces of the puzzle in the story of human evolution, stretching hundreds of thousands of years before today.Shared gene mutations and nucleotide sites of several human populations were searched and analyzed to uncover when the groups diverged as well as the size of the populations that contributed to specific gene pools.
According to Alan Rogers, lead author of the study and professor in the Department of Anthropology, looking for gene mutations of ancient populations in modern humans is like trying to find their fingerprint in other populations – it's only a very small percentage, but it definitely exists.
Rogers and the rest of the research team analyzed the genomes of four human populations, including modern Africans, Modern Eurasians, Neanderthals, and Denisovans.Modern samples were derived from Phase I of the 1000-Genomes project while the archaic samples came from Max Planck Institute for Evolutionary Anthropology.
By analyzing a few million nucleotide sites that shared gene mutation in different population groups, the team was able to establish 10 unique nucleotide patterns.
A new part of the Story
The new study reinforced earlier estimates that modern Eurasians carry about 2 percent of Neanderthal DNA.However, it changed the rest of the human evolutionary story.
The analysis disclosed that 20 percent of nucleotide sites demonstrated a mutation only shared by Denisovans and Neanderthals.This implies a genetic timestamp that marks the period before the separation of the two archaic groups.It was found that the two groups diverged about 744,000 years ago, which is earlier than any conventional estimations.
According to Rogers, if the two groups had separated later, the analysis would show more sites of mutations in the two archaic samples.But that was not the case in the latest study.
The findings also disputed the previous estimation that approximated the Neanderthal population at 1000 individuals.Although Neanderthal DNA has been found to contain mutations that occur in small populations with small genetic diversity, the new study pointed out that Neanderthal remains have been found in various locations and are genetically different.
This is in agreement with the study's finding: that Neanderthals were probably small bands of individuals (which in part explains the harmful mutations), and that the global population was fairly large.The idea is that populations at the archaic period consisted of smaller geographically isolated populations that rarely interacted owing to geographical limitations.
Rogers supports this argument by pointing at the rich Neanderthal fossil record and the many Neanderthal sites that have already been discovered. "It's hard to imagine that there would be so many of them if there were only 1,000 individuals in the whole world," he says.
The research team claims that the latest technique is able to estimate things with high precision farther back in the past, and hopes to apply it in other contexts.