There have been many branches on the hominid tree, but now a lone species walks the earth. We had company once, though, at least in Europe and West Asia - the Neanderthals.
Until recently, the scientific consensus was that they were sufficiently different from Homo sapiens that no interbreeding took place. We knew that they controlled fire; constructed tools, shelters and garments; took care of their weak, injured and elderly; and buried their dead with grave goods. But until two decades ago it was widely believed that they had attributes which disadvantaged them to such an extent that competition with modern humans led to their extinction (for example, lack of capacity for complex language, almost exclusive dependence on hunting for sustenance... to say nothing of the Tarzanist view that their doom came about because -- horrors! -- they allowed women to join the hunt).
This idea of Neanderthals as grunting, shuffling dead-end cavemen began to change as our techniques allowed us to examine Neanderthal fossils with more precision and depth. In the mid-eighties, bone and genetic analysis proved that their ability to hear and produce sounds was almost identical to ours (including a human-like FOXP2 gene, whose function is critical for language). Sequencing of their melanocortin gene indicated that some might have red hair and light skin.
Finally, the just-published draft sequence of their genome (headed by Svante Pääbo's team from the Max Planck Institute) showed that up to 4% of the genes of non-Africans may come from them. If confirmed, this means that Neanderthals and Cro-Magnons indeed interbred when the latter first came out of Africa - most likely in the Middle East, before further migration outward.
The two branches of humanity share 99.7% genetic identity. They show differences in genes involved in cognitive development, skeleton structure, energy metabolism, skin physiology. They also differ in regulatory regions and microRNAs. This information will eventually help us answer the question of what makes us uniquely "human" - perhaps even what has made us so adaptable that we now threaten to overwhelm the earth.
When I read about the conclusions from the draft sequence analysis, tears sprang to my eyes, just as they do at spaceship and planetary probe launches. It moved me inexpressibly to think that they haven't vanished but are with us still, a thread in our fabric, a whisper in our song.
Note 1: The best fictional depiction of the interaction between Neanderthals and Cro-Magnons is Dance of the Tiger by Björn Kurtén, a distinguished vertebrate paleontologist who was Swedish -- as is Pääbo.
Note 2: This article is also on the author's blog, with a nifty graph that shows the genetic interweaving.