Who we are, part 3 (cousins)

When our most distant ancestors first staggered, scampered, or, in the case of the Trumbles, strolled amiably out of Africa about 60,000 years ago, we were not alone. At least two other species of distant hominid cousins walked the Eurasian landmass when Homo sapiens first arrived there, the Neanderthals and the Denisovans. The DNA of all non-Africans is between 1 and 4% Neanderthal in composition, and most of us are approximately 2%. Thanks to Mum, however, my brothers and I are 2.7% Neanderthal, a figure that in my case the exquisite Lyolik reckons is far too low—cheeky! Most indigenous sub-Saharan Africans have no Neanderthal component to their DNA because their ancestors never migrated through Eurasia. Only those whose people left Africa but then turned around and came back again carry any amount of Neanderthal DNA, and they are mostly confined to North Africa, along the southern shores of the Mediterranean Sea.

According to one fairly plausible theory, Neanderthals, Denisovans, and modern humans are all descended from the ancient humanoid Homo heidelbergensis, the original Trumble of us all. Between 300 to 400,000 years ago, an ancestral group of H. heidelbergensis tottered or scurried out of Africa and then split in two shortly thereafter. One branch ventured northwestward into West Asia and Europe and in due course evolved into the Neanderthals. The other branch moved east, becoming Denisovans. By 130,000 years ago, meanwhile, H. heidelbergensis in Africa had evolved into Homo sapiens.

The discovery that our ancient ancestor “cousins” mated with one another could help explain one of the great mysteries in paleoanthropology: Why did the Neanderthals vanish? After first venturing out of Africa long before H. sapiens, Neanderthals thrived in Europe for several hundred thousand years. But about 30,000 years ago somewhat mysteriously they died out, effectively became extinct, in other words at roughly the same moment when we modern Trumbles arrived in Europe. Some scientists have suggested that our H. sapiens ancestors outcompeted or actually clubbed the Neanderthals to death in a colossal struggle for dominance. But new genetic evidence that is enclosed in our own living bodies supports a different and in many ways kinder, better theory: We Trumbles made love and not war with our Neanderthal cousins. The older lineage vanished simply because it was absorbed into the much larger Trumble population through the processes of, at the very least, deep and evidently successful paleolithic flirtation. This comes as no surprise to us; Trumbles are not much given to storms of violence, and indeed we appear to have inherited a genetic predisposition, not always helpful, towards gentleness, even good-natured acquiescence—the better to avoid rocking boats. How charming that this therefore appears to have produced significant evolutionary results, even if, at times, it would not appear to serve us so well in the dog-eat-dog environment of modern life.

Although Neanderthals and Denisovans are both extinct, modern Trumbles may also owe them a mighty large debt of gratitude. A recent study by scientists based at Stanford University concluded that many of us carry ancient variants of immune system genes whose task is to direct our infection-fighting white blood cells to destroy pathogens. Certain of these arose after we left Africa. One very real possibility is that these gene variants entered our DNA as a direct result of mating with our archaic hominid cousins.

Unlike the Neanderthals, the Denisovans are a brand new addition to the human family tree. As recently as 2008, paleoanthropologists digging in a cave in southern Siberia unearthed a 40,000-year-old adult tooth and an exquisitely preserved fossilized finger bone (D5) that once belonged to a young Denisovan girl who was between five and seven years old when she died. Scientists successfully extracted nuclear DNA from that finger bone and compared it with the genomes of modern humans and of Neanderthals. That comparison proved that the girl was closely related to Neanderthals, yet distinct and distant enough from them to merit classification as a new species of archaic human. Scientists named her kin “Denisovan” after the cave where the finger bone was excavated. The Denisovan genome also suggests that this young girl had brown hair, brown eyes, and brown skin.

Surprisingly, these scientists discovered a measure of genetic overlap between the Denisovan genome and that of some present-day east Asians, and, in particular, Melanesians and the peoples of Papua and New Guinea. It appears the Denisovans contributed between 3 to 5 percent of their genetic material to the genomes of Melanesians. It is thought that the most likely explanation for this is that Denisovans living in eastern Eurasia interbred with the direct ancestors of Melanesians. When those humans crossed the ocean to reach Papua New Guinea around 45,000 years ago, they carried their Denisovan DNA with them.

If this genetic mixing did occur, the fact that Denisovans were discovered in Siberia but contributed to the genomes of modern humans living in Southeast Asia suggests the species ranged widely across Asia, although their low genetic diversity also indicates their numbers were never very high.

By comparing the genomes of apes, Denisovans, Neanderthals, and modern humans, scientists hope to identify DNA segments unique to the different groups. Early results already suggest modern humans underwent genetic changes involved with brain function and nervous system development, including ones involved with the evolution of language, after splitting from, or, more likely (in genetic terms) mopping up the Neanderthals and the Denisovans. Identifying and understanding these genetic mutations could help explain why our species survived and thrived while our close relatives died out. But was there something in the character of that process of interbreeding that in fact triggered the quantum leap in brain function, and led in turn to the development of language and all the other remarkable attainments of modern man? We may learn more about this in due course.

The Denisovan component of my Geno 2.0 result is the most experimental, as geneticists are still working to determine the best way to assess the percentage of Denisovan ancestry that we carry. If, as I suspect, the figure of 2.7% Neanderthal would have caused Mum a measure of vexation, she might well have balked at the current estimate of 4.5% Denisovan, though we are told that this figure may well change upward or down as a consequence of further research into many other subpopulations of distant Trumble cousins. Still, all of this is a sobering but delightful reminder that all living people are our kith and kin, and that race is but a mirage of difference. Personally, I am thrilled by the idea that we Trumbles have more in common with those gentle Melanesians than ever we thought possible, for they too are island people.