Genetic secrets of Andaman tribes

ancestral genomes

Jarawas and Onges of the Andaman Islands remain one of the mysteries of science. Over the last 150 years, the inhabitants of the Andaman Islands evoked curiosity among the scholars, who tried to piece together the puzzle on their origin and evolution with limited success. Their origin has been considered different from other Asian population because of their distinctive ‘Negrito’ looks and the unclassifiable language they speak.

Over the last few decades, researchers had suggested that the primitive tribals of the Andaman Islands were the living relic of the first batch of modern humans who moved out of Africa several hundred thousand years ago and travelled along the coast to populate large tracts of Asia, including India. They were those humans, who used the southern exit route, through Ethiopia, the Bab-el-Mandeb Strait and the Arabian Peninsula. Nobody knows how they landed in those islands in the middle of an ocean, but once they did, there were no mix with other ethnic groups.

The ‘Out-of-Africa’ theory, however, does not explain everything as there are serious questions on whether mainland India and Andaman Islands were populated by the same wave of the Out-of-Africa migration. And if there were separate waves, was the wave that populated Andaman Islands substantially earlier than the wave that populated mainland India?

An Indo-Spanish group of researchers has now opened the windows of an exciting new possibility of the Onges and Jarawas to descend from an unknown hominid (ancestor of all great apes and human) that went extinct. There is no fossil record yet, but the DNA analysis provides first evidence on the existence of this new hominid.

Tracing the evolutionary path

“This is the first example of genetics being used to predict the existence of a fossil. Usually, a fossil is discovered first and then studies are undertaken to understand the fossil,” says Partha Majumder, director, National Institute of Biomedical Genomics (NIBMG), Kalyani, West Bengal and one of the lead authors of the study. The research was published in the July 25 issue of Nature Genetics.

The saga of human evolution began almost two million years ago when Homo erectus evolved in Africa. It gave rise to a new species — Homo heidelbergensis about 6,00,000 years ago. About 4,00,000 years ago, Homo heidelbergensis came out of Africa and split into two well known lineages.

One of these lineages gave rise to Neanderthals (Homo neanderthalensis) after moving to Europe through the Middle East. The other lineage likely went east and produced the Denisovans (named after the Denisova cave in Russia from where a finger bone of this species was recovered). Homo heidelbergensis also evolved into Homo sapiens in Africa about 2,00,000 years ago.

Modern humans, Homo sapiens sapiens, largely evolved in Africa about 80,000 years ago. Almost 50,000 years ago, they spread from Africa to different regions of the world and met with other species, such as the Neanderthal and the Denisovan, who looked similar to them. The modern humans, Neanderthals and the Denisovans all shared a common ancestor about 6,00,000 years ago, but it is yet unclear how many different descendant species of this common ancestor existed.

Whatever may the number of such species have been, modern humans don’t seem to have replaced them after they came out of Africa, as the dominant anthropological theory holds. Modern humans mated with them, even if not extensively, as has been inferred from recent DNA evidence. The admixing may have taken place for about 2,500 years.

This was seen from the evidences unearthed by geneticists who sequenced the DNA of the Neanderthal and Denisovans in the last 10 years. Analyses of the DNA sequence data illustrated that when modern humans met with the other species of Homo — the Neanderthal, Denisovan and possibly other — they even interbred. DNA evidence showed that there was recent gene flow from Neanderthals to modern humans after the later migrated out of Africa.

Modern humans came out of Africa in multiple waves. One of the first waves came to India and followed the coastal route to populate the Andaman archipelago. The missing piece of mystery is how did they cross the ocean? But as some of the ethnic groups of the Andaman like the Jarawa and the Onge, look ‘African,’ with dark complexion, short stature and frizzly hair, it was surmised they are a ‘relic’ of the original set of individuals who migrated from out-of-Africa. The yet-to-be-classified language of these Andaman islanders bolstered this belief.

Of different lineages

A recent study reported in the Proceedings of the National Academy of Sciences in February 2016 by the same group of researchers demonstrated how the genetic lineage characterising the Jarawas and Onges was distinct from those characterising the ethnic groups of mainland India. Subsequently, the scientists set out to understand and identify the ancestral components in the genomes of the Andamanese.

The team sequenced the genomes of 60 individuals drawn from a carefully sampled set of diverse ethnic groups of mainland India and 10 Jarawas and Onges. The data was also compared with publicly available population genomics database. The findings are astounding. “When comparing the DNA sequences of the Jarawas and the Onges with those of Neanderthals and Denisovans, we have found some notable differences. By exploring various possibilities that could have given rise to these differences, we have concluded that these DNA fragments belong to an extinct hominid that shares a common ancestor with the Neanderthal and the Denisovan but has a different history,” says Partha.

“Remains of this extinct hominid have not yet been recovered, but our results provide fresh evidence that Homo heidelbergensis had given rise to multiple lineages, not just the Neanderthal and the Denisovan,” he said. The genes also suggest why they are short in height. With 11 out of 107 genes involved in height determination, they concluded that natural selection has favoured the retention of short height among the Jarawas and Onges.