One of the earliest hominins to walk on two legs, Australopithecus anamensis, a near relative of Lucy, is shown by the male's cranium.

Paleoanthropologist Yohannes Haile-Selassie physically jumped for delight when he discovered the entire Australopithecus skull in the Ethiopian sand. Despite seeing many skull fossils, he remarks, "I'd never seen anything like it before."

A 3.8 million year old face has been captured in a stunning photograph thanks to a serendipitous finding by Haile-Selassie and an Ethiopian shepherd. This discovery offers a previously unattainable glimpse at a hominid species from a crucial period in human evolution. The unique fossil, according to experts, can help redraw the evolutionary branches of humans at a time when they were only beginning to develop effective upright walking techniques.

According to Fred Spoor, a specialist in human evolution at the Natural History Museum in London, "This cranium looks set to become another celebrated icon of human evolution," in a News & Views article that was published with Haile-Selassie and colleagues' most recent paper in the journal Nature.

In 2016, the astonishingly complete skull was discovered at Woranso-Mille in Ethiopia's Afar region. But it has taken three and a half years of arduous work to respond to the initial query—exactly what type of skull is it?

The skull, known as MRD after a portion of its collection ID number, was examined by Haile-Selassie and colleagues with a wide range of hominin fossils from all around Africa. To determine what species the skull represents and where it fits in the interwoven lineages of our family tree, they evaluated various morphological traits. The findings show that a male Australopithecus anamensis was the owner of the skull. According to theory, the hominid species went extinct somewhat earlier than 3.8 million years ago after giving rise to Australopithecus afarensis, a subsequent branch that includes the famous specimen Lucy. A. anamensis possesses characteristics that are shared by both apes and humans, such as climbing arms and wrists and altered ankle and knee joints that allow for two-footed walking.

The majority of A. anamensis fossil specimens previously discovered have only been little fragments of bone, such a tooth, a portion of a jaw, or an arm or shin piece. The chance to examine a virtually intact braincase and face verifies the existence of a distinct species known as the "southern ape" and sheds light on the distinctions between A. anamensis and A. afarensis, two of the most prehistoric hominin progenitors.

The majority of A. anamensis' own characteristics, according to Haile-Selassie, are relatively primitive, including the creature's small brain, projecting face, and huge canine teeth. "Some characteristics, such as the orbital zone in the frontal area, are shared only with A. afarensis. However, everything else is incredibly archaic. It resembles an ape when viewed from the back. I never anticipated to see something like this in a species that is thought to be the ancestor of A. afarensis. Therefore, it altered a wide range of conceptions regarding their relationship.

The skull also challenges common beliefs that the older lineage gave rise to the younger one directly by indicating that the two may have coexisted for at least 100,000 years. However, the authors of the study emphasize that it's still very likely that early populations of A. anamensis gave rise to A. afarensis around 4 million years ago—they just didn't disappear right away.

"It is likely that a small population of A. anamensis separated from the dominant population, suffered significant modifications, and gradually separated itself from the parent species of A. anamensis. That is most likely how A. afarensis emerged", according to Haile-Selassie.

The research team contends that the relationship between the two extinct hominin species, thought to be the ancestors of our own genus Homo, may be an excellent illustration of a nonlinear evolutionary process typical of other non-human animals. Our family tree's branches did not diverge primarily due to anagenesis, which occurs when one species transforms into another so completely that its parent vanishes.

"Just because one species gave rise to another, it doesn't mean that the source species (ancestor) disappeared," writes Rick Potts, director of the Smithsonian's Human Origins Program, via email from a dig in Kenya. Potts was not involved in the new study. "Like the evolutionary trees of nearly all other species, we have long known that the human family tree is branching and diversified. The new cranium is important because it demonstrates this pattern of variety in a little-known stage of hominin evolution, at the same time as our forebears became increasingly committed to standing up straight."

According to research by paleoanthropologist Meave Leakey and colleagues, A. anamensis was the first species to develop an enlarged knee joint that allowed each of its legs to momentarily support the entire weight of the animal during bipedal locomotion. With the ability to walk on two legs, ancient hominins were able to exploit a larger variety of habitats than those accessible to tree climbers, setting them distinct from the apes.

By examining the minerals and volcanic layers where the cranium fossil was discovered, a second, related study was able to more precisely date it. A. anamensis and his ancestors' long-gone environment was also described in the paper.

On the banks of an old lake, the skull was buried in sand that had been deposited in a river delta. Additionally, the sediment deposits contained evidence of plants, showing that a variety of other local ecosystems coexisted with the dry shrubland that made up the area around the old lake.

According to Beverly Saylor, a geologist at Case Western Reserve University and the principal author of the second research, "the area was dry with few trees, but there were forests around the shores of the lake and along the river that flowed into it." The data points to the male hominin presumably consuming a hard, ape-like diet of seeds, grasses, and similar foods, like contemporaries from other sites.

For fifteen years, Haile-Selassie and colleagues have worked in the Ethiopian region of Woranso-Mille. Haile-Selassie was dubious when a local shepherd announced to the camp that he had discovered some intriguing fossils, especially considering how frequently locals had brought him to alleged fossil locations just because they needed a ride somewhere. He requested that the shepherd accompany him on an hour-long trek to the location of his find from Habib Wogris, the regional leader who coordinates fieldwork in the area each year.

According to Haile-Selassie, "The chief has seen a lot of hominin teeth from the site and he realized that this tooth looked like a hominin tooth." I questioned where he had found the tooth as soon as he returned and opened his palm. Let us go, and we will show you, they said.

The shepherd had relocated his flock to the fossil site in the region's high terrain in order to avoid seasonal flooding in lower areas. Haile-Selassie explains that the man had been living there with his goats for around three months when he discovered the fossil while he was creating a shelter for the young goats from jackals and hyenas.

The shepherd led him to the spot where the tooth had been, and Haile-Selassie looked around for further pieces.

Haile-Selassie says, "Three meters from where I was standing there was this round thing, just like a rock, and I said oh my goodness." He literally started bouncing up and down in response, which led the shepherd to remark that the doctor had gone insane. I told them, in their language, that the doctor was not crazy. Haile-Selassie chuckles, "He is just excited.

Since the rare fossil was first discovered three years ago, scientists have been eager to put a human, or hominin, face on our distant relatives. With its official unveiling today, the enthusiasm has extended around the scientific community.