By Katie Hunt, CNN
(CNN) — A prehistoric human known as Homo erectus was the first of our forerunners to leave Africa, crossing continents and ultimately roaming the planet for almost 2 million years. But with scarce genetic material available to study, the species remains a major mystery in human origins.
Now, scientists have retrieved ancient proteins from six teeth unearthed in China that, for the first time, reveal a molecular link between Homo erectus and later human species, including our own: Homo sapiens.
“This is a major step forward in tying together the broken branches of our human evolutionary tree,” said Ryan McRae, a paleoanthropologist at the Smithsonian National Museum of Natural History in Washington, DC, who was not involved in the study. “Homo erectus has long been a bit of an enigma.”
Homo erectus remains have been found in Africa, Asia and Europe; however, obtaining informative molecular data such as DNA has proved challenging given the fossils’ age and poor preservation.
In a study published Wednesday in the scientific journal Nature, Chinese geneticist Fu Qiaomei and her colleagues successfully extracted and analyzed ancient enamel proteins from the teeth unearthed at three sites in China. All the teeth date from around 400,000 years ago.
Proteins, which are made up of sequences of amino acids, are more robust than ancient DNA, a fragile molecule that degrades relatively easily. Proteins contain far less detailed information, but they can still shed some light on a specimen’s evolutionary history.
Fu, a professor at the Institute of Vertebrate Paleontology and Paleoanthropology, part of the Chinese Academy of Sciences in Beijing, and her team used what they described as a new, less invasive technique to study fossils without damaging their morphology.
Rather than drilling, they used acid etching to remove a small sample of enamel from the teeth. The team did not try to recover DNA from the fossils after failing to extract DNA from animal fossils of the same age from the same sites. Fu said it was hard to get DNA, but she would never give up.
Unknown variant discovered
The researchers found that the specimens from the three sites in China shared two amino acid variants, one of which was previously unknown. This finding, the researchers reasoned, suggested the teeth all belonged to the same species.
The second variant had previously been identified in Denisovans, another shadowy species of ancient human, and also in some modern human populations.
That other human species shared this variant suggested that Denisovans had once interbred with Homo erectus, and then, at some later point, Denisovans mated with Homo sapiens, according to the study.
As a result, traces of Denisovan DNA live on in some humans today — something interbreeding geneticists call admixture.
Similarly, modern human populations have some Neanderthal ancestry — a legacy of past interactions with that species that went extinct about 40,000 years ago. Denisovans also interbred with Neanderthals.
Modern human populations in Southeast Asia have the highest Denisovan ancestry, suggesting the two groups once crossed paths there.
‘Ghost lineage’
Geneticists knew that Denisovans had some ancestry from an unknown “ghost lineage” with no DNA match, and Homo erectus was one possible candidate, said Eduard Pop, a research scientist at Naturalis Biodiversity Center in Leiden, Netherlands, via email.
“This study strengthens