Another "All Your Yesterdays" entry: Denisovan, or "Polar Neandertal"

They've extended the deadline for the All Your Yesterdays contest, so I've decided to do a couple more entries. I started this one years ago as a Neandertal restoration. Since that time, new genomic discoveries showed that the speculative pigmentation was incorrect. But, other discoveries identified a new candidate for the subject matter!

Known from a few scrappy pieces, the Siberian Denisovans (Homo sp. or Homo sapiens ssp., depending on how large you like your species) are a true challenge to reconstruct. We have their entire genome, but know almost nothing about their anatomy. The few fossil elements we have are not morphologically distinct from Neandertals (Homo neanderthalensis or Homo sapiens neanderthalensis) or humans (Homo sapiens sapiens).

But the genomic facts are highly intriguing:

1. Some Oceanian humans have inherited up to 6% of their nuclear DNA from Denisovans (with the highest ratios in Meganesia [Australia and New Guinea]). 
2. The nuclear DNA indicates a common ancestor with Neandertals, shortly after the split from proto-humans.
3. But the mitochondrial DNA indicates a motherline that branched off much earlier. (Possibly Homo erectus?)
4. Genes for pigments are consistent with dark skin.

Here I've imagined a Siberian Denisovan as a sort of "polar Neandertal". As with polar bears, his skin is dark, trapping heat, but his pelage is light, allowing for camouflage against the taiga and tundra. He is the last of his kind — his southern kin mixed with the strange, baby-faced people who keep invading from the west. But he does not welcome them. He will fight to his death.

Human Clades: A Look at a Complex Phylogeny

Most methods of phylogenetic analysis deal with simple trees. In these phylogenies, every taxonomic unit has a single direct ancestor (or "parent"). But we know that phylogeny is often more complex than this. Our own species is an excellent example—while we are all primarily descended from one population in Africa, different peoples around the globe have inherited smaller percentages of ancestry from preexisting populations.

A new study by Reich & al. looks in some detail at peoples who have inherited DNA from the Denisovans, a fossil group known from Siberia. Ancient DNA has been retrieved from these fossils, although unfortunately the fossils are otherwise too scant to tell us much about what Denisovans looked like (other than "humanlike").

Reich & al. posit a complex phylogeny wherein populations are often descended from multiple ancestral populations. Lets take a look at the clades posited in this study.

Operational Taxonomic Units

Reich & al. used the following nine populations, seven extant and two extinct, as operational taxonomic units.

Yoruba.—An ethnicity from West Africa (Nigeria, Benin, Ghana, etc.)
(Photo by Marc Trip.)

Han.—The most populous Chinese ethnicity.
(Photo by Brian Yap.)

Mamanwa.—One of the "Lumad" ("indigenous") ethnicities of the southern Philippines.
(Photo by Richard Parker.)

Jehai.—One of the Orang Asli ("original people") groups of Malaysia.
Note: this photo is of a woman from a different Orang Asli tribe, the Batik.
(Photo by Wazari Wazir.)

Onge.—A group of Andaman Islanders, from the Bay of Bengal.
(Photo from The Andamanese, by George Weber.)

Australians.—The indigenous ("aboriginal") peoples of Australia.
(Photo by Rusty Stewart.)

Papuans.—The indigenous peoples of the New Guinean highlands.
(Photo owned by the Center for International Forestry Research.)

Neandertals.—An extinct group of robust near-human peoples from West Eurasia.
(Photo by myself, of a sculpture by John Gurche.)

Denisovans.—An extinct group of near-human peoples known from Siberia but thought to have had a wider range.
Note: The photo is of a sculpture of Homo heidelbergensis, thought to be the common ancestor of humans, Neandertals, and Denisovans. Denisovans may not have looked exactly like this.
(Photo by myself, of a sculpture by John Gurche.)

Phylogeny

Reich & al. postulated the simplest phylogeny that could possibly explain their data. (Note that the actuality is likely more complex than this, but it's a good starting point.) More recent groups are to the right, and the thickness of the lines indicates the percentage of DNA contributed from population to population.

My diagram, not theirs. Any inaccuracies are my own.
Free for reuse under Public Domain.

I've added a line for the Denisovans' mitochondrial (motherline) ancestor, even though it's not part of the paper's phylogeny. More on that as we start looking through the various clades.


For looking at the clades I'll use a different diagram that does not reflect percentage of ancestry, but simply shows direct descent as unweighted arcs connecting parent and child taxonomic units.

Phylogeny of human and near-human populations according to Reich & al. 2011.
Created using Names on Nodes.
Free for reuse under Public Domain.