We all have in us the DNA of our ancestors. Finding those ancestors, though, can take a lot of work.
Ryan Bohlender is a statistical geneticist with the MD Anderson Cancer Center in Houston. He spends much of his time trying to unlock the history of human DNA. Late last month, he revealed research results that may provide another stunner about the origins of humans: a previously unknown and now extinct relative of modern man.
Not Neanderthals exactly. Not the recently discovered (but long extinct) species of man known as Denisovan. This was something else — technically, some one else — probably with Neanderthal and Denisovan influence in that same branch of the human family tree.
The secret to this potential find was hidden in the DNA of living people from Melanesia, an area in the South Pacific near Papua New Guinea, just north of Australia. Bohlender and his colleagues compared their DNA to the DNA of some of our ancestors, then ran the numbers against a few statistical models to come up with their hypothesis.
"There are sort of immediate reasons why it's important," explains Bohlender, who detailed his findings at the annual meeting of the American Society of Human Genetics in Vancouver. "There have been several papers recently about the effect of Neanderthal DNA. The Neanderthal DNA that you carry with you is associated with everything from obesity to depression to smoking risk. There are health consequences of Neanderthal DNA. So exploring what this is, where it came from, how much of it anybody might have, that sort of thing, is practically useful.
"In reality, the reason why a lot of us do it is because we're curious about where we all came from. And it's cool. It's an anthropological question, right? This is about the history of who we are and how we got here and what makes us what we are today."
Back in Time
To understand Bohlender's findings, you should know a little about how modern man got to this point, evolutionarily speaking.
The Smithsonian Institute says there are as many as 15 to 20 different species of early man. Homo sapiens — modern man — is the only one left, sitting at the top of the tree. (The Smithsonian sketches out a nice-looking tree here.) Neanderthals — let's call them brothers from another species of mother — are in the same big area of the tree (as are Denisovans), though they may be best described as being on a different branch.
This general area of the tree represents the first group of hominids to expand beyond Africa, the birthplace of humans. The earliest members of the group lived between 2 and 3 million years ago. Homo sapiens popped up around 200,000 years ago.
All of this, widely accepted in the scientific community, is the result of decades of study by archaeologists, paleontologists, anthropologists, paleoanthropologists and others. More recently, geneticists like Bohlender have begun mapping modern man's genes and comparing them to what we know of those who preceded us.
The Melanesian Mystery
In March of 2010, a finger bone of a formerly unknown human ancestor, later called Denisovan, was found in a Siberian cave where modern human remains and Neanderthal remains also were found. A few years later, scientists declared that human ancestors and Neanderthals interbred and that many living humans still carry Neanderthal DNA.
When Bohlender and his colleagues looked at the DNA of modern Melanesians, the people from Papua New Guinea, they estimated what percentage of Neanderthal DNA and Denisovan DNA they had. The number was much less than what other researchers had estimated. After more calculations and more computer modeling, Bohlender and his crew laid out one potential answer: the possibility of a third, unknown human ancestor contributing to the Melanesian gene pool.
"On the one hand, it's possible that we're missing a branch. That there is another archaic lineage, potentially related to Denisova, potentially older, potentially related to Neanderthal," Bohlender says. "We're not specific about where this missing branch is."
A missing branch, of course, could alter the thinking of everyone who looks into human origins. But Bohlender is careful to point out other possibilities. The unknown species might simply be an offshoot of a species that migrated to Melanesia and became genetically different over time.
Whatever the case, it's clear that a simple tree — though it makes for a nice graphic — may not be entirely accurate forever. As in all things human, things get complicated, but when has that ever stopped a scientist?
"The tree itself that's relating Neanderthal and Denisovans to modern humans is probably pretty good right now. And the big hominid tree of life going back all the way to Australopithecus is probably pretty accurate," Bohlender says. "We're going to find more fossils, more specimens, more little branches that may or may not have led to us as time goes on. But I think the general story is pretty solid, and has been for a while."