The face of the oldest species that unambiguously sits on the human evolutionary tree has been revealed for the first time by the discovery of a 3.8 million-year-old skull in Ethiopia.

The fossil belongs to an ancient hominin, Australopithecus anamensis, believed to be the direct ancestor of the famous “Lucy” species, Australopithecus afarensis. It dates back to a time when our ancestors were emerging from the trees to walk on two legs, but still had distinctly ape-like protruding faces, powerful jaws and small brains, and is the oldest-known member of the Australopithecus group.

While Lucy became celebrated in studies of human evolution, her direct predecessor has remained a shadowy trace on the record, with only a handful of teeth, some limb bones and a few fragments of skull to provide clues about appearance and lifestyle.

The latest specimen, a remarkably complete adult male skull casually named MRD, changes this.

Quick guide Evolutionary timeline Show Hide 55m years ago First primates evolve. 15m years ago Hominidae (great apes) split off from the ancestors of the gibbon. 8m years ago Chimp and human lineages diverge from that of gorillas. 4.4m years ago Ardipithecus appears: an early "proto-human" with grasping feet. 4m years ago Australopithecines appeared, with brains about the size of a chimpanzee’s. 2.3m years ago Homo habilis first appeared in Africa. 1.85m years ago First "modern" hand emerges. 1.6m years ago Hand axes are a major technological innovation. 800,000 years ago Evidence of use of fire and cooking. 700,000 years ago Modern humans and Neanderthals split. 400,000 years ago Neanderthals begin to spread across Europe and Asia. 300,000 years ago Evidence of early Homo sapiens in Morocco. 200,000 years ago Homo sapiens found in Israel. 60,000 years ago Modern human migration from Africa that led to modern-day non-African populations.

“It is good to finally be able to put a face to the name,” said Stephanie Melillo of the Max Planck Institute for Evolutionary Anthropology, based in Germany, who is the co-author of an analysis of the find.

Prof Fred Spoor of the Natural History Museum, London, who was not involved in the research, said the discovery of MRD – and its dating to a period when the fossil record is very sparse – would substantially affect thinking on the evolutionary family tree of early hominins. “This cranium looks set to become another celebrated icon of human evolution,” he said.

The skull shows that MRD had a small brain – about a quarter of the size of a modern human – but was already losing some of its ape-like features. Its canines are smaller than those seen in even earlier fossils and it is already developing the powerful jaw and prominent cheekbones seen in Lucy and the famous Mrs Ples fossil (another later member of the Australopithecus group), which scientists think helped them chew tough food during dry seasons when less vegetation was available.

Facebook Twitter Pinterest The 3.8m-year-old skull. Photograph: Michael Tewelde/AFP/Getty Images

The dating of the skull also reveals that Anamensis and its descendent species, Lucy, coexisted for a period of at least 100,000 years. This discovery challenges the long-held notion of linear evolution, in which one species disappears and is replaced by a new one. Anamensis, which now spans from 4.2 million to 3.8 million years ago, is still thought to be Lucy’s ancestor, but continued to hang around after the Lucy group branched off from the parent lineage. Geological evidence suggests the landscape would have featured extremely steep hills, volcanoes, lava flows and rifts that could easily have isolated populations, allowing them to diverge.

Divergent groups may have later crossed paths and competed for food and territory.

Yohannes Haile-Selassie, of the Cleveland Museum of Natural History and Case Western Reserve University, who led the research, said: “This is a game changer in our understanding of human evolution during the Pliocene.”

Afarensis, which continued to appear on the fossil record until at least 3 million years ago, has often been put forward as a likely candidate ultimately giving rise to the Homo lineage to which modern humans belong. But the discovery that multiple different lineages coexisted makes this hypothesis much less certain, according to the researchers.

“Having multiple candidate ancestral species in the right time and place makes it more challenging to determine which gave rise to Homo,” said Melillo.

Facebook Twitter Pinterest Yohannes Haile-Selassie with the skull. Photograph: AFP/Getty Images

Spoor described Anamensis as the “oldest-known species that is unambiguously part of the human evolutionary tree”. Older fossils, like Ardi, which dates to 4.4 million years, are more contentious – some say it is on the human lineage, while others regard it as an extinct form of ape.

The first piece of the new fossil, the upper jaw, was found by a local worker in February 2016, in the Afar region of Ethiopia. “I couldn’t believe my eyes when I spotted the rest of the cranium. It was a eureka moment and a dream come true,” said Haile-Selassie.

Fossil pollen grains and chemical remains of fossil plant and algae taken from the sediment suggest that the individual lived by a river or along the shores of a lake surrounded by trees and shrubland.

The findings are published in the journal Nature.