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Thursday, 10 September 2015

A New Human Ancestor Arises From The Depths Of A South African Cave

Lateral view of the skull of DH1, a specimen of the newly-named hominin species Homo naledi. (Image by John Hawks; CC BY 4.0.)
In 1964, Louis Leakey and colleagues proposed the name ofHomo habilis, the “handyman,” for fossil remains of a human ancestor in Tanzania, making it the earliest species in a genus that includes us, Homo sapiens. This morning, Lee Berger and colleagues announced the discovery of another long-lost relative: Homo naledi.  Intriguingly, this new species presents a skeleton with some features that are surprisingly modern and some features that are surprisingly archaic. But since it has not yet been securely dated, how exactly H. naledi fits into our evolution is still a bit murky.
Two years ago, a team of experts from around the world was assembled in order to document and collect hundreds of fossils found in the Dinaledi chamber of the Rising Star cave system in South Africa. Spelunking nearly 100 feet underground, a portion of the Rising Star Expedition team squeezed through narrow crevices, bringing in cameras to beam images of the fossils to the surface before removing them. The collection they amassed — totalling 1,550 specimens of a hominin, or human ancestor — represents at least 15 different individuals of the same species, astoundingly high numbers in palaeoanthropology, the field that studies human evolution. More to the point, these remains are all similar and yet “include traits not found in any other hominin species yet described,” Berger and colleagues write today in the journal eLIFE.
Naming a new species is not undertaken lightly, and the decision to do so is based on distinctive features and appearances of fossil remains. Berger and colleagues compared the fossil bones and teeth found in Dinaledi cave to other known hominin specimens, including the robust and gracile australopithecines, Homo habilisHomo erectusand ergaster, Homo floresiensis (or the “hobbit”), and “archaic” Homo sapiens, in an attempt to figure out if the Dinaledi remains could be classified with these already known species. Some of the most interesting bones in this case are those of the hands and feet.
Hand 1. Palmar view on left; dorsal view on right. This hand was discovered in articulation and all bones are represented except for the pisiform. The proportions of digits are humanlike and visually apparent, as are the expanded distal apical tufts on all digits, the robust pollical ray, and the unique first metacarpal morphology.
Figure 6 from Berger et al. 2015, eLIFE. Palmar view of right hand of Homo naledi. This hand was discovered in articulation, and all bones are represented except for the pisiform. The proportions of digits are humanlike and visually apparent, as are the expanded distal apical tufts on all digits, the robust pollical ray, and the unique first metacarpal morphology. (Image by John Hawks, CC BY 4.0.)

Hands are important in human evolution because our opposable thumbs are considered a key “derived” trait, or one that differs from the ancestral pattern. Our ability to grasp everything from a tree branch to a pencil is owed to our ancestors’ evolution. What’s intriguing about the hand of the new Homo naledi is that it “possesses a combination of primitive and derived features not seen in the hand of any other hominin,” according to Berger and colleagues. In short, the shape of the first metacarpal, the bone that runs along the base of the thumb, means that H. naledi had a muscular, human-like hand.
Feet are also important in human evolution because we, unlike other primates, walk on them exclusively. Far back in our evolution, changes had to occur over time to our ancestors’ skeletons to make bipedal walking efficient and useful. The foot bones of these new fossil individuals have much more in common with us than they do with older australopithecines and early members of our genus Homo.They do, however, have much lower arches than our feet do.
Two views (superior and lateral) of the rearticulated right foot of Homo naledi. (Image by Peter Schmid, CC BY-SA.)
Two views (dorsal and medial) of the rearticulated right foot of Homo naledi. (Image by Peter Schmid, CC BY 4.0.)
But while Homo naledi boasts human-like hands and feet, as well as a body size similar to a small human and a skull of a similar shape, its brain was small and it has a ribcage, shoulders, and pelvis that are decidedly not similar to ours. Berger and colleagues call the skeletal structure of Homo naledi an “anatomical mosaic,” meaning the new species has some features we expect to see in more ancient species and some features we expect to see in more recent species of hominin. “This species combines a human-like body size and stature with an australopith-sized brain,” they write, as well as “features of the shoulder and hand apparently well-suited for climbing with humanlike hand and wrist adaptations for manipulation; australopith-like hip mechanics with humanlike terrestrial adaptations of the food and lower limb; small dentition with primitive dental proportions.”
The key features of the Dinaledi fossils, though, place them within our genus. That is, the way H. naledi walked, grasped objects, and chewed food is more similar to our Homoancestors than to earlier australopithecines. “The similarities of H. naledi to earlier members of Homo,including H. habilisH. rudolfensis, and H. erectus,” the authors explain, “suggest that this species may be rooted within the initial origin and diversification of our genus.”
While H. naledi has not yet been securely dated, the shape of its skeleton likely places it early in our genus. “If the fossils prove to be substantially older than 2 million years,” Berger and colleagues note, “H. naledi would be the earliest example of our genus that is more than a single isolated fragment.” But if they’re more recent, a date “younger than 1 million years ago would demonstrate the coexistence of multiple Homo morphs in Africa, including this small-brained form, into the later periods of human evolution.” No matter the date, the unique form of these skeletons will start filling in gaps in the fossil record of early humans.
Dinaledi skeletal specimens. The figure includes approximately all of the material incorporated in this diagnosis, including the holotype specimen, paratypes and referred material. These make up 737 partial or complete anatomical elements, many of which consist of several refitted specimens. Specimens not identified to element, such as non diagnostic long bone or cranial fragments, and a subset of fragile specimens are not shown here. The ‘skeleton’ layout in the center of the photo is a composite of elements that represent multiple individuals. This view is foreshortened; the table upon which the bones are arranged is 120-cm wide for scale. (Image by John Hawks, CC BY 4.0; text from Figure 1 of Berger et al. 2015, eLIFE)
Dinaledi skeletal specimens. The figure includes approximately all of the material incorporated in this diagnosis, including the holotype specimen, paratypes and referred material. These make up 737 partial or complete anatomical elements, many of which consist of several refitted specimens. Specimens not identified to element, such as non diagnostic long bone or cranial fragments, and a subset of fragile specimens are not shown here. The ‘skeleton’ layout in the center of the photo is a composite of elements that represent multiple individuals. This view is foreshortened; the table upon which the bones are arranged is 120-cm wide for scale. (Image by John Hawks, CC BY 4.0; text from Figure 1 of Berger et al. 2015, eLIFE)
It is clear from the amazing work done in just two years by the Rising Star Expedition that the bar has been raised in palaeoanthropology, particularly in terms of classifying and naming new species in the future. The mosaic-like set of skeletal features represented in the hundreds of fossil remains from Dinaledi cave means that “we must abandon the expectation that any small fragment of the anatomy can provide singular insight about the evolutionary relationships of fossil hominins,” Berger and colleagues caution. Mounting evidence of the evolution of different traits at different times in different populations means the fossil record of human evolution is much more complicated — and much more interesting — than we thought even a few decades ago.
The Dinaledi cave is in a way unique, having protected one of the largest and most comprehensive collections of hominin remains anywhere in the world, but Berger and his team are confident similar assortments exist elsewhere. They conclude their article by admitting that, while we do not yet know what the entirety of the Homo ancestral tree looked like, “with an increasing pace of discovery from the field and the laboratory, more light will be thrown on the origin of humans.”

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