Recent analyses of fossil DNA have revealed that modern humans occasionally had sex and produced offspring not only with Neanderthals but also with Denisovans, a relatively newfound lineage whose genetic signature apparently extended from Siberia to the Pacific islands of Oceania.
This year, hints began emerging that another mystery human lineage was part of this genetic mix as well. Now, the first high-quality genome sequence from a Neanderthal has confirmed those suspicions.As Dennis Venema points out, modern humans appear to come from a population of around 150,000, which likely arose in North Africa. I would still suggest two possibilities that might explain the hybridization. One, that from around this time to 50 thousand years ago, humans existed as a generally polytypic species and that interbreeding, until the demise of the Neandertals and the Denisovans was, perhaps, widespread. In the 50 thousand years since then, these genes have stochastically dropped out of the population.
These findings come from Denisova Cave in southern Siberia, where the first evidence of Denisovans was discovered in 2008. To learn more about the Denisovans, scientists examined DNA from a toe bone unearthed there in 2010.
The researchers found that the fossil belonged to a Neanderthal woman. Her DNA helped refine the human family tree, as it revealed that about 1.5 to 2.1 percent of the DNA of modern people outside Africa is Neanderthal in origin, whereas about 0.2 percent of DNA of mainland Asians and Native Americans is Denisovan in origin. [Top 10 Mysteries of the First Humans]
The other possible model that would explain the variation between Neandertals, Denisovans and early moderns might be a syngameon, in which there are three definable species that, at their peripheries, hybridize. At some point, then, the modern humans simply swamped the other two genomes. This would be the model of modern-day dogs and wolves which can mate but ordinarily do not.
Another finding? That early Homo may have been one highly-variable species, rather than a bush of species:
The level of variation seen in Homo fossils is typically used to define separate species. However, analysis of 1.8-million-year-old skulls excavated from the Republic of Georgia revealed the level of variation seen among those skulls was about the same as that seen among ancient African Homo fossils. As such, researchers suggest the earliest Homo fossils may not be multiple human species, but rather variants of a single lineage that emerged from Africa.This will rattle those that hold to more cladistically-driven models of human origins like Ian Tattersall who supports a model more like the phylogenetic species concept, in which single trait difference might confer species-level taxonomy. This takes care of this issue of why there are two very different morphs on the landscape in East Africa at this time, represented by ER 1813 and ER 1470. The dimorphism is quite large, but, as we discovered, is not beyond what would be expected in any species of large-bodied hominoid but larger than what would be found in any living population of humans in the last several hundred thousand years.
A good run-down. Read the whole thing.