The evolution of man from primitive Australopithecines to modern hominids like Homo habilis and Homo
erectus has always seemed pretty straightforward, at least if you believe those old ape-to-man
evolution charts from high school.
But in fact, the path from Australopithecus africanus to Homo sapiens is complicated.
And the more we uncover about our DNA, the more we have to adjust the story of our evolution
to match the facts, including the weird discovery of a mysterious "ghost population" of missing
ancestors.
Scientists now believe that ancient hominids were, ahem, indiscriminate lovers.
Genetic artifacts from the free loving Stone Age suggest that our ancestors all took it
where they could get it, and that meant sometimes straying outside the tribe.
Yes, homo habilis got down with its contemporary, Homo erectus, as well as other cousins roaming
the savannah.
Even Homo sapiens, yes, us, enjoyed looking for opportunity in the Neanderthal camps.
Anatomically modern humans are the only surviving species of the Homo genus populating the planet
today, but the hominid dating pool wasn't always so homogenous.
Our direct genetic line diverged from our closest living relatives, bonobos and chimpanzees,
somewhere between 4 million and 7 million years ago.
During most of that time, multiple species of hominids roamed the planet simultaneously.
These species were so closely related that they were capable of producing fertile offspring.
Scientists know that these disparate populations copulated, because some of their genes survive
today in the deep recesses of our genome.
Biologists can usually peg these genes to known species of hominid, but they've been
stumped by a recently discovered pocket of code from the DNA of modern West Africans.
This code appears to derive from some "ghost population" of undiscovered hominids populating
the area around 500,000 years ago.
According to best estimates, these missing ancestors split off from a Neanderthal precursor
species between 360,000 and 1 million years ago.
A population of approximately 20,000 likely interbred with ancestors of modern West Africans
as recently as 124,000 years ago.
So, genetically speaking, this so-far-undiscovered relative split off from modern humans even
earlier than the Neanderthals.
According to researcher Sriram Sankararaman:
"They seem to have made a pretty substantial impact on the genomes of the present day individuals
we studied.
They account for 2% to 19% of their genetic ancestry."
While 19% may seem like quite a bit of genetic material, evolutionary biologists are still
unsure what, if any, impact these kinds of contributions have on modern humans.
One UCLA researcher explained that,
"while several studies have revealed contributions from deep lineages to the ancestry of present-day
Africans, the nature of these contributions remains poorly understood."
So while we see the genes right there in our modern genome, we're not sure yet what they
exactly do.
But researchers are trying to find out.
In the latest study, researchers compared 405 individual genomes from West African participants.
They cross-referenced their results with other known hominids, like Neanderthals, to determine
whether or not their data suggested a new, undiscovered genetic contributor.
Some experts say this result isn't unexpected.
There were likely many different human populations interbreeding during this epoch, each with
distinguishable bits of genetic code.
Professor Joel D. Irish of Liverpool John Moores University said,
"I think at one time, there'd have been all sorts of populations, with genetics different
enough to look a bit different.
Everybody tends to mate with everybody.
I think we're going to find more and more of these 'ghost' populations coming up."
The evidence certainly suggests he may be right.
West Africans certainly aren't the only modern humans that show contributions from other
species of hominid.
Western European samples show significant percentages of Neanderthal DNA, while indigenous
Australians also have contributions from their local ancestors, the Denisovans.
But with other hominids gone, the era of interbreeding is over.
Bonobos and chimpanzees are too distantly related to modern humans to productively mate,
so opportunities for this kind of hybridization no longer abound.
So what’s the reason we humans cleared the field of all our closest relatives?
The best guess is that our advanced brains and capacity for language probably helped
us out-compete all the other hominids occupying the same ecological niche, leading to their
extinction.
So let’s pour one out for our extinct cousins.
May their contributions live on in our DNA!
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