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[THREAD] Researchers working in Ethiopia have discovered a nearly complete fossil cranium from a long-vanished member of the human family and it is NEXT LEVEL 🤩. Here’s the lowdown. [Image: Dale Omori, courtesy of Cleveland Museum of Natural History] 
The fossil (nature.com/articles/s4158…), dated to 3.8 million years ago (nature.com/articles/s4158…), finally reveals the face of Australopithecus anamensis, a human species first described in 1995.
Previously anamensis was known mainly from jaws, teeth and a few bones from below the head (scientificamerican.com/index.cfm/_api…)
Traits evident in the specimen hint that our family tree (scientificamerican.com/article/the-or…) may need redrawing. [Image: illustration by Katy Wiedemann for Scientific American]
By some accounts, Australopithecus anamensis is the oldest unequivocal member of the hominins (the group that includes Homo sapiens and its close extinct relatives), with some fossils dating back as far as 4.2 million years ago (nature.com/articles/natur…)
Other, older fossils unearthed since then—including Sahelanthropus from Chad, Orrorin from Kenya and Ardipithecus from Ethiopia--have been put forth as hominins, but skepticism has dogged those claims (scientificamerican.com/article/an-anc…).
Not only is anamensis quite old, but for years it has occupied a key position in the family tree as the lineal ancestor of Australopithecus afarensis, the species to which the celebrated Lucy fossil belongs (blogs.scientificamerican.com/observations/4…).
Many experts think afarensis is the ancestor of our own genus, Homo, although other candidates have been proposed over the years (scientificamerican.com/article/first-…).
Based on the ages and characteristics of the available fossils, scientists thought anamensis gave rise to afarensis through an evolutionary process termed anagenesis in which one species gradually transforms into another. The new fossil throws a wrench into that scenario.
A team led by paleoanthropologist Yohannes Haile-Selassie of the Cleveland Museum of Natural History recovered the cranium from an area in northeastern Ethiopia’s Afar region known as Woranso-Mille. [Image: Courtesy of the Cleveland Museum of Natural History)
Features of its teeth and jaws link it to the previously known fragmentary remains of anamensis. The fossil, dubbed MRD for the Miro Dora locality in which it was found, also preserves much of the face and braincase—critical aspects of the anatomy that have never been seen before
MRD reveals a creature with a projecting face, large canine teeth, flaring cheekbones, a crest on its head that anchored strong jaw muscles, and a long, narrow braincase that held a brain the size of a chimp’s. [Image: Dale Omori, courtesy of Cleveland Museum of Natural History]
The discovery team suspects the cranium belonged to an adult male anamensis. Paleoartist John Gurche reconstructed the face of this individual. [Image: Matt Crow, courtesy of the Cleveland Museum of Natural History]
Here’s how it could upend conventional wisdom: based on the anatomy revealed by MRD, Haile-Selassie and colleagues argue that an enigmatic 3.9 million-year-old forehead bone found in 1981 at another site in the Afar region called Belohdelie belongs not to anamensis but afarensis.
If they are right, this means that anamensis, which is now known from fossils spanning the time between 4.2 and 3.8 million years ago, and 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 imply that anamensis could not have evolved into afarensis by means of anagenesis. Instead afarensis split off from anamensis, which continued to exist for a time alongside its daughter species. This branching mode of evolution is known as cladogenesis.
Cladogenesis can occur when populations of a species become isolated from one another, allowing them to evolve in different directions.
At a press teleconference on Tuesday, geologist Beverly Saylor of Case Western Reserve University said that MRD lived in a very diverse setting that might have led to such isolation of populations.
But the case for cladogenesis over anagenesis hinges entirely on that 3.9 million-year-old forehead bone from Belohdelie belonging to afarensis—no other afarensis remains recovered thus far are that old.
And problematically, with only one anamensis forehead bone to compare it to—MRD’s—one cannot exclude the possibility that other anamensis individuals might've had foreheads that looked like the Belohdelie one.
Another open question is whether anamensis is the ancestor of afarensis (the scenario Haile-Selassie's team favors) or something else. In some respects MRD resembles younger hominins such as Australopithecus africanus and species in the genus Paranthropus.
Haile-Selassie and his co-authors chalk these similarities up to parallel evolution. But might anamensis be the ancestor of these hominins rather than afarensis? If so, who gave rise to afarensis?
For that matter, who is the ancestor of anamensis? (Also: could similarities between MRD and Sahelanthropus and Ardipithecus help support their status as hominins?) I can’t wait to find out!
