This month, a new fossil analysis out of East Africa shows that several kinds of humans coexisted about 2 million years ago, near the dawn of all human species.
Scientists say the familiar cartoon image of increasingly upright creatures progressing to a human male is misleading. We should picture a bushy tree, with multiple branches or types of humans existing at different points in time. Many early models of humanity were not our ancestors, but simply died out.
Even after Homo sapiens emerged, we coexisted with Neanderthals as well as a lesser-known group called the Denisovans, the long-lived species Homo erectus, and the diminutive "hobbit" species Homo floresiensis. The last group didn't go extinct until about 12,000 years ago, an eyeblink in evolutionary time.
"There have been many experiments in becoming human, most of which didn't succeed in the long run," said William Kimbel, who directs the Institute of Human Origins at Arizona State University.
It's weird to be alone in a genus, said anthropologist Daniel Lieberman of Harvard University. There are multiple species of squirrels, bats, whales, alligators, ducks, and many other animals. "What is actually bizarre is that there's only one species of us," he said.
The most recently verified addition to the human family is Homo rudolfensis, whose status as a separate species was debated until three new fossils emerged from Kenya. They date from between 1.9 million and 1.75 million years ago, early in the history of the genus Homo (and well before the emergence of our species).
One of the known human species to go back that far is called Homo habilis. Human origins pioneer Louis Leakey discovered the first H. habilis skeleton in 1972. It dated to 1.9 million years ago, and was given the status of homo because it walked upright, and has been associated with stone tools. (The common name for H. habilis is handy man.)
Soon, however, other African fossils from the same time period turned up with a different set of features - a flatter, bigger-boned face and a larger brain. Did they represent a new species or two sexes of H. habilis?
Those who favored the different-species hypothesis named the bigger ones Homo rudolfensis.
The problem was they didn't have enough fossils to conclusively establish a distinct species. There were lots of fragments and only one complete skull of the alleged second species, H. rudolfensis.
"If you have a sample of one, you can never convince the skeptics," said Meave Leakey of the Turkana Basin Institute, which is associated with Stony Brook University. (Leakey is herself part of a complex tree of famously related anthropologists - she's married to Louis Leakey's son Richard, and their daughter Louise was a collaborator on the fossil find.)
With too little fossil evidence, the question remained unresolved until Meave Leakey and her colleagues found the additional specimens in Kenya - two lower jaws and a well-preserved face from a juvenile.
The fossils appear to be remains of H. rudolfensis, Meave Leakey said, making a solid case that it was a distinct species from H. habilis. The analysis was published this month in the journal Nature.
Which species, if either, was our ancestor remains the subject of debate.
And we should be careful using bones to make declarations about species, said ASU's Kimbel. In biology a species is defined not by the way skeletons look but by such things as gene exchange and reproductive independence. That can be complicated, since closely related species can interbreed to some extent. DNA evidence suggests a bit of hanky-panky went on between our ancestors and Neanderthals, for example, but we weren't all one species, he said, because the interbreeding didn't erase the distinctiveness of the Neanderthals.
Just how "human" were these early members of the Homo family? They used stone tools and made a living as hunter-gatherers, said Harvard's Lieberman, just as H. sapiens has for most of our existence. They walked upright and were physically capable of long-distance running - an ability that he thinks played a key role in our uniqueness.
Exactly when our lineage became human may always be a judgment call, said ASU's Kimbel, because there are a number of characteristics we use to define ourselves - cooperative hunting, tools, language, division of labor, and art, to name a few. These were acquired piecemeal over hundreds of thousands of years, he said.
The scientists don't yet have a complete explanation for how we ended up alone in our genus. Something about our behavior or physiology allowed us to outcompete our closest evolutionary relatives, said Lieberman, or maybe we just got lucky.
What we do know is that every day we also get closer to becoming the only member of our wider taxonomic grouping - the so-called superfamily Hominoidea, which includes chimpanzees, bonobos, orangutans, and gorillas. Then, barring contact with intelligent aliens, we will be even more alone.
Contact Faye Flam at 215-854-4977 215-854-4977 or firstname.lastname@example.org, or follow on Twitter @fayeflam. Read her blog at philly.com/evolution.