For much of modern history, human evolution was thought to progress in linear fashion, with one species replacing another, one by one. The famous ‘ape turning to human’ cartoon illustrating a stooped primate gradually straightening into a modern man essentially captures this idea.
That picture began to change in the second half of the 19th century when scientists started to unearth human-like fossils. At the time, scientists used geological clues such as the depth at which a fossil was found and the rock layers surrounding it to determine the age of fossils. This method could establish which fossils were older or younger than others but did not reveal precise dates.
The timeline suggested an intriguing possibility, however. It appeared that these different kinds of humans had not followed one another in succession but had lived alongside each other. Radiometric dating techniques later confirmed this idea: multiple human species had coexisted on the earth at various points.
Series of surprises
In 2003, the Human Genome Project announced the first high-quality sequence of the human genome. Human beings finally possessed the secret to their own identity — the reason(s) they were unique. That sense of identity proved to be short-lived.
In 2010, scientists published the genome of the Neanderthals, our closest extinct relatives. When they compared Neanderthal DNA with the genomes of living people, they uncovered a remarkable result. Most humans alive today carry around 1-2% Neanderthal DNA in their own genomes. In Africans, this percentage is slightly lower, but even their DNA is 0.3-0.5% Neanderthal.
The surprises did not end there. In 2012, scientists sequenced the genome of Denisovans, another human lineage, finding that some present-day populations, particularly in Oceania and parts of Southeast Asia, had 3-6% Denisovan DNA.
That we carry DNA fragments from two other members of the genus Homo raised the possibility that we may also harbour genetic material from other extinct human species. But in the 14 years since the Denisovan genome was published, no new genome from any other extinct human relative has been uncovered.
DNA after death
The difficulty lies in the nature of DNA itself. The moment an organism dies, its DNA begins to break down. Enzymes released from dying cells cut DNA into small pieces. Microbes and fungi invade the remains and further degrade the genetic material. Water and oxygen promote chemical reactions that further damage DNA. Then there are temperature fluctuations, solar radiation, and UV damage. Over a few tens of millennia, the DNA is reduced to a few small pieces.
To recover DNA from these ancient humans, scientists need to be very lucky. The specimen must have been exposed to cold, dry, and stable environments such as permafrost, frozen sediments, or the inside of a deep cave. This is very rare — and why only a small fraction of fossils contain usable DNA.
One such species whose DNA sequence has evaded us for a long time is Homo erectus, the species estimated to have originated over 2 million years ago and thought to be one of the first human relatives to spread widely across Africa, Europe, and Asia.
Acid etching
A recent study published in Nature has provided the first molecular sequences from Homo erectus fossils from China. Rather than attempting to recover DNA directly, the researchers extracted proteins preserved within the enamel of six Homo erectus teeth dated to around 400,000 years ago. Because proteins are the products of the DNA, their sequences can reveal portions of the underlying genetic information, providing a rare, albeit narrow, view into the genome of an extinct human species.
The method used to recover the genetic information is notable. Usually, recovering ancient DNA or proteins requires scientists to grind up part of a fossil, so museum curators are reluctant to permit such analyses, especially when there is no guarantee that usable genetic material will be recovered.
To overcome this problem, the researchers used acid etching, a technique where a small area of the tooth enamel is briefly exposed to a dilute acid solution, which dissolves microscopic amounts of tooth enamel and releases the proteins trapped within it. The fossil is left largely intact. Since tooth enamel is a highly mineralised tissue, it can trap and protect proteins for a long time.
The researchers proceeded to successfully recover enamel proteins from five male and one female Homo erectus individuals. When they compared the protein sequences with those of modern humans, Neanderthals, and Denisovans, they made two interesting discoveries.
First, all six Homo erectus individuals had a protein variant that has never been found in any other known species in the genus Homo. Second, they carried another variant present in Denisovans. This suggests populations related to these Chinese Homo erectus may have interbred with Denisovans in East Asia.
Shared story
However, because the results come from only the enamel proteins and not the entire genome, they are not conclusive. It was long known that the Denisovan genome contains DNA from a much older species of the human lineage. While this study points to Homo erectus as a possible candidate, it does not prove it.
Nevertheless, the study represents an advance far beyond the information it has generated, which by itself is tiny. This is the first effort to recover meaningful molecular data from Homo erectus fossils, long thought to be beyond the reach of genetics.
Though Neanderthals, Denisovans, and others have vanished from the earth, traces of their existence endure within our DNA. The more we look, the more we find that the story of humans is not only ours: it is theirs as well.
Arun Panchapakesan is an assistant professor at the Y.R. Gaitonde Centre for AIDS Research and Education, Chennai.
Published – June 08, 2026 07:30 am IST
