The Smithsonian Institution’s Human Origins Initiative counts seven species as belonging to the genus Homo. But that’s just a fraction of all the species that scientists have proposed for our genus. Over the years, as researchers have realized fossils from different groupings actually come from the same species, anthropologists have tossed out the names that are no longer valid. Last spring, I highlighted several of these now-obscure names, as well as some recently proposed species that are not universally accepted. Here’s a look at four more proposed species of Homo that you probably won’t find in human evolution text books or museum exhibits.
Homo antiquus: In 1984, Walter Ferguson of Israel’s Tel Aviv University declared that Australopithecus afarensis wasn’t a real species (PDF). At the time, the known fossils of A. afarensis came from the site of Hadar in Ethiopia and Laetoli in Tanzania. There was a lot of physical variation among the bones in this combined collection, but many anthropologists thought the diversity was simply due to size differences between male and female members of the species. Ferguson, however, believed the bones actually represented more than one species. Based on the size and shape of the molars, Ferguson concluded that some of the larger jaws at Hadar matched those of Australopithecus africanus, a species that had only been found in South Africa. Other jaws in the collection had smaller, narrower Homo-like teeth, he said. The roughly three-million-year-old fossils were too ancient to fit with any of the previously described members of the genus Homo, so Ferguson created a new species name—H. antiquus. Ferguson’s species splitting had a larger implication: If Australopithecus and Homo had lived side by side for hundreds of thousands of years, it was unlikely that australopithecines were the direct ancestors of Homo. Ferguson’s work must not have been convincing. Almost 30 years later, A. afarensis is still around and few people have ever heard of H. antiquus.
Homo kanamensis: Many of Louis Leakey’s discoveries have stood the test of time. H. kanamensis is not one of them. In the early 1930s, Leakey unearthed a hominid lower jaw at the site of Kanam, Kenya. The jaw resembled those of modern people in many ways, but was thicker in some places. Leakey determined the jaw should have its own name: H. kanamensis. At about half a million years old, the species was the oldest member of Homo yet found—except, the fossil wasn’t really that ancient. Subsequent geological studies at Kanam revealed that the jaw was only a few tens of thousands of years old. And the jaw’s unusual thickness was due to an abnormal growth, suggesting H. kanamensis was nothing more than a diseased Homo sapiens.
Homo capensis: In the early 1910s, two farmers stumbled across hominid fossils, including bits of a skull, near Boskop, South Africa. The bones were passed around to many anatomists—including Raymond Dart, who later discovered the first Australopithecus fossil—before ending up in the hands of paleontologist Robert Broom. Broom estimated the brain size of the skull (PDF): a whopping 1,980 cubic centimeters (the typical modern person’s brain is around 1,400 cubic centimeters). Broom determined that the skull should be called H. capensis, also known as Boskop Man. Other specimens from South Africa were added to the species, and some scientists became convinced southern Africa was once home to a race of big-brained, small-faced people. But by the 1950s, scientists were questioning the legitimacy of H. capensis. One problem was that the thickness of the original skull made it difficult to estimate the true brain size. And even if it were 1,980 cubic centimeters, that’s still within the normal range of variation for modern people’s brains, anthropologist and blogger John Hawks explained in 2008. Another problem, Hawks pointed out, was that scientists were preferentially choosing larger skulls to include in H. capensis while ignoring smaller skulls that were found in association with the bigger specimens. Today, fossils once classified as H. capensis are considered members of H. sapiens.
Homo rhodesiensis: If you have heard of any of the species on this list, it’s probably this one. Paleontologist Arthur Smith Woodward created the name H. rhodesiensis for a skull discovered in 1921 at Broken Hill, or Kabwe, in Zambia (once part of Northern Rhodesia). The fossil’s thick skull, sloped forehead and giant brow ridges made the species distinct from living people. Other robust African fossils dating to around 300,000 to 125,000 years ago were added to the species. However, this group of fossils has been known by many other names. Some anthropologists, for example, think the bones belong to early, more archaic members of our own species, H. sapiens. However, most researchers today lump H. rhodesiensis fossils with the more widespread species Homo heidelbergensis, which lived in Africa and Eurasia starting roughly half a million years ago and may have been the common ancestor of modern humans and Neanderthals.