HUMANS & SOCIE TY
Debate over Homo
Fossils’ age and arrival in
cave remain mysterious
BY BRUCE BOWER
Homo naledi, a rock star among fossil
species in the human genus, has made
an encore. Its return highlights debate
over whether this hominid was a distinct
Homo species that purposefully disposed
of at least some of its dead.
H. naledi made worldwide headlines
last year when researchers announced
the discovery of an unusually large collection of odd-looking Homo fossils in
the bowels of a South African cave system. Presentations on April 16 underscored key uncertainties about the
One of the biggest mysteries: H. naledi’s
age. Efforts are under way to date the
fossils and sediment from which they
were excavated with a variety of techniques, said paleoanthropologist John
Hawks of the University of Wisconsin–
Madison. An initial age estimate may
come later this year if different dating
techniques converge on a consistent figure. A solid date for the fossils is essential for deciphering their place in Homo
evolution and how the bones came to
rest in a nearly inaccessible cave.
Some presenters reasserted that
H. naledi intentionally dropped dead
comrades into an underground chamber, where their bones were later found
by cave explorers and then scientists.
But others raised questions. Even team
leader Lee Berger, a paleoanthropologist
at the University of the Witwatersrand
in Johannesburg, hedged his bets.
“It’s way too early to tell how H. naledi
bodies got in the chamber,” Berger said.
Berger’s group recovered 1,550
H. naledi fossils from a minimum of
15 individuals of all age groups (SN:
10/3/15, p. 6). Slender researchers
wended through narrow passageways
in South Africa’s Rising Star cave sys-
tem and squeezed down a vertical chute
to reach pitch-dark Dinaledi Chamber.
There, they found hominid fossils scattered on the floor and up to 20 centimeters beneath the surface.
Berger’s team assigned the bones to a
new species called H. naledi based on an
unexpected mix of humanlike features
and traits typical of Australopithecus species from more than 3 million years ago.
Fossil analyses presented at the meeting challenged a suggestion by some
researchers, both before and during the
meeting, that H. naledi actually represents a variant of Homo erectus, a species known to have existed by 1. 8 million
years ago (SN: 11/16/13, p. 6).
H. naledi possessed a shoulder unlike
those of other Homo species, said team
member Elen Feuerriegel of the Australian National University in Canberra.
H. naledi’s collarbone and upper arm
bone resemble corresponding
Australopithecus bones, she reported. H. naledi’s
shoulder blades must have been positioned low and behind the chest, an
arrangement more conducive to climbing trees than running long distances.
H. naledi’s hand was built both for
climbing and gripping stone implements, said Tracy Kivell of the University of Kent in England. Her analysis
of 150 hand bones, including a nearly
complete hand, showed a humanlike wrist and thumb combined with
Australopithecus-like curved fingers.
H. naledi’s curved toes and flaring pelvis also recall Australopithecus.
Still, a preliminary lower-body reconstruction — incorporating fossil evidence of humanlike legs, knees and
feet — suggests H. naledi walked almost
as well as modern humans do, said Zach
Throckmorton of Lincoln Memorial
University in Harrogate, Tenn.
Homo naledi’s hands (one
shown from two views)
wrists and thumbs with
curved fingers characteristic of tree-climbing
hominids. This hand design sets H. naledi apart
from other Homo species,
H. naledi and West Asian H. erectus
share several tooth features as well as
relatively small braincases and similar statures. “That complicates matters,” said Christopher Walker of Duke
University. Upper-body features that
Berger’s team considers characteristic of H. naledi, such as the upper arm’s
shape, possibly occurred in West Asian
H. erectus as well, added Tea Jashashvili
of Witwatersrand, who has studied those
Explaining how H. naledi bones ended
up in Dinaledi Chamber is also complicated. Ongoing studies of sediment and
rock indicate that there was never a
direct opening to the underground fossil site from above, said Marina Elliott of
Bones from some body parts, including five feet, three hands and part of a
backbone, were found aligned as they
would have been in living individuals,
indicating at least some bodies reached
the chamber intact, Hawks said. Curiously, some sets of aligned bones were
found beneath scattered bones from
If the dead were dropped down a vertical chute into Dinaledi Chamber, bodies
on top would have been least damaged
and most likely to retain aligned bones.
Along with that mystery, some sets of
aligned bones somehow ended up far
from the chute’s opening, Berger said.
An alternative entrance to Dinaledi
Chamber possibly existed in the past,
Aurore Val, of Witwatersrand, asserted
online March 31 in the Journal of Human
Evolution. Beetles or snails that damaged
some H. naledi bones don’t inhabit dark,
underground caves, Val argued. Such
damage probably occurred on the sur-
face or in a nearby, once-accessible part