Year in review: Early human kin could shake up family tree

Scientists trying to untangle the human evolutionary family’s ancient secrets welcomed a new set of tantalizing and controversial finds this year. A series of fossil discoveries offered potentially important insights into the origins of the human genus, Homo. Most notably, a group of South African fossils triggered widespread excitement accompanied by head-scratching and vigorous debate.

If the discoverers of the South African fossils are right about what they have found, then at least some early members of the Homo genus possessed an unexpected patchwork of humanlike and apelike features, with legs and feet built for upright walking but shoulders, chests and hips suited to climbing trees. These ancient hominids had brains much smaller than anyone expected, housed in skulls shaped like those of later Homo species. In the year’s most intriguing evolutionary development, Lee Berger of the University of the Witwatersrand in Johannesburg and colleagues reported finding 1,550 fossils from a previously unknown species that they call Homo naledi (SN: 10/3/15, p. 6).

After noticing fossils on the floor of an underground cave in South Africa, cave explorers alerted Berger to the find. Requests on Twitter and Facebook located six researchers with spelunking experience; each recruit was a slender woman who could fit through a narrow cave passage and navigate a final 12-meter descent into the pitch-black chamber. The unusual nature of the effort received global attention, showing that the ways people explore the past can be as compelling as the specifics of what they find.

Once in the chamber, the intrepid band recovered bones from at least 15 individuals. Their brains would have been no larger than those of 2-million- to 4-million-year-old hominids from the genus Australopithecus .

Humans descended directly from one particular Australopithecus species, says Brian Villmoare of the University of Nevada, Las Vegas. In Ethiopia, he and his team discovered what may be the oldest known Homo fossil, dating to 2.8 million years ago (SN: 4/4/15, p. 8). The partial jaw shares features with fossil jaws from Australopithecus afarensis, a hominid species that died out in Ethiopia around 3 million years ago. Villmoare and colleagues suspect that A. afarensis, which includes Lucy’s famous partial skeleton, evolved into the human genus.

As expected in a field that deals with partial remains of long-extinct species, not everyone agrees with Berger’s or Villmoare’s conclusions. Shifting soil layers in the cave where H. naledi was found make it difficult to find and date the original location of the fossils, so scientists don’t know how old the bones are. While Berger contends that H. naledi probably inhabited Africa’s southern tip more than 2 million years ago, near the time the Homo genus originated, it’s also possible the fossils are younger. They might belong to a previously known species, Homo erectus, or even represent an Australopithecus species from that same time period. As for the ancient Ethiopian jaw, critics say it’s hard to draw any conclusions without more bones.

There’s one big discovery this year that scientists can agree on: The making of stone tools originated before the Homo genus did. Sonia Harmand of Stony Brook University in New York led a project that unearthed 3.3-million-year-old stone implements in Kenya (SN: 6/13/15, p. 6), clear evidence that East African hominids from Lucy’s era made them too. Until Harmand’s report, stone tools had been dated to no more than about 2.6 million years ago.

Debate continues as more fossils are unearthed. Researchers from the Cleveland Museum of Natural History, for example, recently discovered what they are describing as a new species of Australopithecus that lived alongside Lucy (SN: 6/27/15, p. 7). Others plug that find into Lucy’s species. Either way, the back and forth isn’t keeping the researchers down. They’ll just have to drag a few more skeletons out of the closet.