Scientific American - USA (2019-12)

December 2019, ScientificAmerican.com 59

PHOTOGRAPHY BY DALE OMORI AND LIZ RUSSELL; COMPOSITE IMAGE OF HANDS HOLDING “MRD” BY JENNIFER TAYLOR AND CLEVELAND MUSEUM OF NATURAL HISTORY

NEARLY 25 YEARS AFTER SCIENTISTS

described the first fossil traces of Australo­

pithecus anamensis, this unsung human ances-

tor is finally having its moment. Researchers

working in Ethiopia have found a  nearly com-

plete cranium of this long-vanished member

of the hominin group, which includes Homo

sapiens and its close extinct relatives. The

fossil, dated to 3.8  million years ago, reveals

the never before seen face of A. anamensis,

a  species previously known mainly from jaws,

teeth and a smattering of bones from below

the head. Traits evident in the specimen hint

that our family tree may need revising.

By some accounts, A. anamensis is the oldest unequivocal

hominin, with some fossils dating from as far back as 4.2  million

years ago. For years it has occupied a key position in the family

tree as the lineal ancestor of Australopithecus afarensis, which is

widely viewed as the ancestor of our own genus, Homo. Based on

the ages and characteristics of the available fossils, paleoanthro-

pologists thought A. anamensis gave rise to A. afarensis through

an evolutionary process termed anagenesis, in which one species

transforms into another. The new fossil throws a wrench into the

works of that theory.

Yohannes Haile-Selassie of the Cleveland Museum of Natural

History and his colleagues recovered the cranium from an area in

northeastern Ethiopia’s Afar region known as Woranso-Mille.

Features of its teeth and jaws link it to the previously known frag-

mentary remains of A. anamensis. The fossil shows a creature

with a projecting face, large canine teeth, flaring cheekbones, a

crest atop its head that anchored strong jaw muscles, and a long,

narrow braincase that held a brain the size of a chimpanzee’s.

The discovery team suspects the cranium belonged to an adult

male A. anamensis.

Here is how it could upend the conventional wisdom: on the

basis of the more complete A. anamensis anatomy seen in the

newly discovered cranium, Haile-Selassie and his colleagues ar-

gue that an enigmatic 3.9-million-year-old forehead bone from

the site of Belohdelie, also located in Ethiopia’s Afar region, be-

longs to A. afarensis. If this supposition is right, A. anamensis,

which is known from fossils spanning the time between 4.2  mil-

lion and 3.8  million years ago, and A. afarensis, which apparently

lived from 3.9 million to 3.0 million years ago, actually overlapped

for at least 100,000 years in the Afar. And that overlap would im-

ply that A. anamensis could not have evolved into A. afarensis by

means of anagenesis. Instead A. afarensis split off from A. ana­

mensis, which continued to exist for a time alongside its daughter

species. This branching mode of evolution, known as cladogene-

sis, can occur when populations of a species become isolated from

one another and are thus able to evolve in different directions.

But the case for cladogenesis over anagenesis hinges entirely on

that 3.9-million-year-old forehead bone from Belohdelie belonging

to A. afarensis —no other A. afarensis remains recovered thus far

are that old. Problematically, with only one A. anamensis forehead

bone to compare it with—the one in the new fossil—one cannot

exclude the possibility that other A. anamensis individuals might

have had foreheads that looked like the Belohdelie one. Only dis-

covery of more fossil faces can resolve that unknown.

A FACE FROM

DEEP TIME

A long-sought fossil cranium could redraw the human family tree

By Kate Wong

PALEOANTHROPOLOGY

Kate Wong is a senior editor

for evolution and ecology

at Scientific American.

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