The last changes that made the difference, says S.Ananthanarayanan.
Humans and other mammals are said to share an ancestor of some 80 million years ago. As various body functions are the same in all mammals, the make-up of the DNA is also largely the same across species. And this is especially true of the part that of the DNA that codes for proteins.
A question that arises is, how much closer are we to our nearest relatives, the chimpanzees, or even the earliest, but extinct, humans, or hominids, like the Neanderthals or the Denisovans. Lucia Coppo, Pradeep Mishra, Nora Siefert, Arne Holmgren, Svante Pääbo, Hugo Zeberg, from Karolinska Institutet, Stockholm, Max Planck Institute for Evolutionary Anthropology, Leipzig and Okinawa Institute of Science and Technology, Japan, in the AAAS journal, Science Advances, go into a specific genetic difference between modern humans and an extinct, close evolutionary relative, and find that it may have given humans a survival advantage.
While the human genome is fully mapped, and we can do the same with chimpanzees, or other living things, it is not straightforward in the case of an extinct species. Here, the genetic information has to be extracted from ancient DNA that is in bits of tissue, bone or teeth, often found in fossilized remains. And this is challenging because the material in the samples is miniscule, and what is there gets contaminated, over time and in handling. The method to deal with scarcity of DNA is to amplify what is available, using the polymerase chain reaction (PCR), a technique that has become better known with current testing for COVID-19. And contamination, at least the part that arises during handling, can be minimized through care and special methods and procedures.
The first recovery of DNA from the remains of an extinct creature was in 1984, from a museum specimen of the quagga, a zebra-like animal last seen a century earlier. A few years later, with the help of PCR, bits of DNA were recovered of 7,000 year-old, soft human brain tissue, found in peat bogs in florida. PCR is a powerful tool, which makes it possible to work with very small traces, even in bones and teeth. The problem of contamination, some of which was by the PCR process, however, still remains. Nevertheless, with innovation, like multiple trials and statistical methods, it became possible to sequence the DNA of a 30,000 to 1,00,000 yr-old hominid. And with still more improvements, a draft sequence of the genome of a 38,000 yr-old hominid was published in 2010.
The 38,000 yr-old remains were of a Neanderthal human, an extinct sub-species first identified in 1856, from bones discovered in a limestone cave in Neandertal, a small valley in North Rhine-Westphalia. The bones were recognized as different from those of modern humans, but it took half a century before the Neanderthals were considered a legitimate species.
Another strain of early human forerunners were the Denisovans, so named after the discovery of ancient remains in the Siberian Denisova cave. The bones found were of a young female and DNA was extracted from a finger bone, as recently as 2010. The Denisovan DNA compares well with the Neanderthal and it appears that Neanderthals also inhabited the cave, but it is not clear if the two streams met and interbred.
“Neanderthals and Denisovans, so-called “archaic” humans, shared an ancestral population with the ancestors of modern humans about half a million years ago,” the paper in Science Advances says. This would be the point, A, in the diagram, where modern humans separated. And, as parts of the DNA of the Neanderthals and Denisovans have been accurately sequenced, it is possible to “identify the changes that characterize modern humans, the paper says.
Genetic change consists of accidental changes in the structure of units of the DNA. These units, the nucleotides, are of four kinds, the difference being which of four ‘bases’, A, C, T or G, the nucleotide contains. Along the lineage leading to humans, we know of about a hundred instances, where the base in one of the units has been changed, which are in the DNA of most humans, but not in the known DNA portions of Neanderthals and Denisovans, the paper says.
While Neanderthals and Denisovans arose in Europe and Asia, the line that has come down as humans originated in Africa. This line migrated out of Africa and became dominant. When the line left Africa and encountered Neanderthals, the paper says, there was interbreeding and some Neanderthal DNA variants were introduced into the human gene pool. And these markers of contact with Neanderthals can be detected to this day, the paper says.
A challenge faced by oxygen dependent organisms, like mammals, is to overcome cell damage by a reactive form of oxygen. The way animals down the evolutionary chain have managed is with the help of the anti-oxidant, glutathione. Glutathione consumes reactive oxygen, which saves important cell components. And then, it is recycled, with the help of an enzyme, glutathione reductase, or GR.
And one of the 100-odd genetic differences between humans and Neanderthals, the paper says, is one that affects the form of GR. The variant in most humans has the amino acid, glycine, in the sequence, but the Neanderthals have the amino acid, serine in place of glycine. And as a result of the gene flow from Neanderthals to humans, the Neanderthal form, or the ancestral form, of GR is found in a small proportion of humans, 1 – 2 % in the Indian subcontinent and much less in Europe, the paper says.
Analysis shows, the authors say, that the ancestral form of GR is associated with disease of the circulatory system and with irritable bowel syndrome, or IBD. Both conditions are associated with oxidative stress – overproduction of reactive oxygen causes inflammation of blood vessel membrane and reduced availability of glutathione is a characteristic of IBD, the paper says. And further, GR is found in many parts of the body and oxidative stress is the reason for other disease too. The modern, human form of GR may hence play a protective role in more areas than the two that have been identified.
It is possible, the paper says, that this difference in the form of GR gave humans an advantage, which enabled them to dominate, while the so-called archaic humans went extinct.
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