An international research team has announced the most complete fossil yet of Homo habilis (aka 'the handy man') – one of the earliest known members of our genus.
The 2-million-year-old partial skeleton may even represent the oldest example of H. habilis discovered so far.
It includes a nearly full set of teeth, as well as ancient bone fragments from the shoulder blades, arms, ribs, pelvis, upper leg bones, and cranium.
The precious fossils were found in the last decade or so, scattered in geological layers in northern Kenya that date to between 2.02 million and 2.06 million years old. Scientists have carefully analyzed and reassembled each and every part.
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"There are only three other very fragmentary and incomplete partial skeletons known for this important species", says lead author Fred Grine from Stony Brook University in the US.
These fossils have quite a broad age range, with the most recent a jawbone dating back just 1.44 million years ago.
This newly described H. habilis fossil solidly sits in the two-million-year age range.
H. habilis is famous for being one of the oldest archaic human species to make stone tools, branching off from the Australopithecus genus to which the renowned Lucy fossil belongs.
In fact, H. habilis is often considered a crucial bridge between the tree-swinging Australopithecus genus and the Homo genus, which led to our two-legged savannah-running ancestors.
More than three million years ago, in southern Africa, Lucy stood just over a meter tall (3.6 feet) and weighed just 29 kilograms (64 pounds).
H. habilis arose a million or so years later, and the species seems to have had a larger braincase than Lucy, but with a smaller face and smaller teeth.
The finger bones on H. habilis also point to the evolution of precision grip, a key human trait that might have been used for tool making or to prepare meat.
For around half a million years, the range of H. habilis overlapped with that of another hominin species, called Homo erectus, a species named for an upright posture that probably allowed it to travel efficiently on two legs. Whether H. habilis swung from the trees or also walked remains controversial.
The newly described H. habilis fossil, named KNM-ER 64061, has longer and stronger arm bones than those of H. erectus. The fossil is also shorter and lighter, standing approximately 160 centimeters tall and weighing between 30.7 and 32.7 kilograms.
The anatomical features could be a sign that H. habilis spent more time in the trees than on the ground, but that idea remains speculative.
Unfortunately, the KNM-ER 3735 specimen is too poorly preserved to properly evaluate the proportions of its arms compared to its legs.
"What remains elusive is the lower limb build and proportions," explains Ashley S. Hammond, ICREA Researcher at the Catalan Institute of Palaeontology Miquel Crusafont, who joined the study team in 2014.
"Going forward, we need lower limb fossils of Homo habilis, which may further change our perspective on this key species."
Without that extra information, Hammond and colleagues say they are hesitant to infer how this particular individual lived.
Based on the fossil's teeth and cranial bones, however, the research team concludes that this was almost certainly a young adult.
That so much of its skull has remained is remarkable. So far, only two cranial sections with associated dental remains have been found for Homo erectus and three for Homo habilis.
Recent compelling evidence suggests that both species co-existed in eastern Africa between 2.2 and 1.8 million years ago. Plus, several other hominin species probably lived at this time and in the same region: P. boisei and H. rudolfensis.
The human lineage did not evolve in a straight line, but this newly described fossil gets us closer to the beginning of our tangled family tree.
The study was published in The Anatomical Record.