This is a tale of two skeletons. It is the saga of a pair of ancient members of the human family from Ethiopia nicknamed Lucy and Ardi. The former is an icon of early humanity while the latter is lesser-known, but no less important and perhaps more revelatory. Their stories reveal much about early human evolution – and how the science of our past has advanced over the last half century.
The Afar Depression of Ethiopia is one of the most productive fossil-producing regions of the world. Part of the East African Rift System, this sedimentary basin was formed by the separation of continental plates. Thanks to favourable geology, its sun-scorched deserts represent a prime hunting ground for extinct members of the human family.
The potential of this region came to light in the 1970s thanks to the pioneering work of geologist Maurice Taieb. After finding the ground strewn with petrified bones, he invited French and American scientists to form a research team and they quickly focused on one fossil-rich area called Hadar.
In 1974, anthropologist Donald Johanson and his graduate assistant Tom Gray found Lucy, a 3.2 million year-old skeleton. When reconstructed, the pieces composed about 40 per cent of the skeleton (or 70 per cent after lab technicians created mirror image replicas of bones missing on the opposite side) of a petite female with an ape-sized brain who stood just over 1 metre tall.
The Hadar team collected hundreds more specimens of the same species later dubbed Australopithecus afarensis. These filled in parts missing from Lucy, including skull, hands, and feet. Today this fossil species is one of the best-known in the human family with more than 400 specimens ranging from 3 to 3.7 million years old.
The discovery of Australopithecus afarensis advanced science in numerous ways.
First, it illuminated one of the greatest mysteries of humanity: why did our ancestors stand upright? Humans resemble our primate cousins in many aspects of anatomy, but we are bizarrely unique when it comes to our two-legged locomotion.
Darwin had theorized that humans evolved erect posture in tandem with stone tools, big brains, and small canine teeth, but afarensis showed that these traits did not evolve as a package. Rather, upright locomotion began long before big brains and stone tools.
Second, these discoveries pushed the human fossil record deeper into the past and established the genus Australopithecus as a viable ancestor to our genus, Homo. (The genus is one taxonomic rank above the species and typically unites taxa that share a common adaptive niche).
After much debate, little doubt remains that Lucy’s species were bipeds. Australopithecus afarensis had straight big toe – not a grasping one – and the beginnings of a humanlike arched foot (despite having more primitive foot proportions than we do). This species is the likely suspect to have left the humanlike footprints in fossilised volcanic ash at Laetoli, Tanzania 3.6 million years ago.
This does not necessarily mean Lucy’s species had abandoned the trees entirely; it retained some features that some scholars interpret as evidence of climbing including curved fingers and toes, mobile shoulder joints, and long forearms.
But what came before Lucy – and how did bipedality begin? Beyond 4 million years ago, the fossil record of our ancestors remained almost entirely blank for two decades after the discoveries at Hadar.
In 1992 in another part of the Afar Depression known as the Middle Awash, an American-Ethiopian team based at the University of California at Berkeley picked up the first pieces of a primitive species more than 1 million years older than Lucy. The early finds included diamond-shaped canine teeth, distinct from the dagger-like fangs of apes, which marked these creatures as primitive members of the human family.
In 1994, the Middle Awash team hit an unexpected jackpot – a 4.4 million year-old skeleton of a species named Ardipithecus ramidus. Ethiopian scholar Yohannes Haile-Selassie found a broken hand bone, triggering an intensive search and the discovery of more than 125 pieces of an ancient female who stood about 1.2 meters tall with a grapefruit-sized brain of about 300 cubic centimetres.
Nicknamed Ardi, the skeleton preserved many parts missing from Lucy (including hands, feet, and skull) and was 1.2 million years older. Searchers eventually found more than 100 specimens from other individuals of this species.
Shortly after the Ardi skeleton had been transported back to the lab, paleoanthropologist Tim White made a shocking discovery – Ardi had a grasping big toe of a tree climber. This revelation arrived alongside seemingly contradictory ones; Ardi’s other four toes displayed anatomy similar to upright bipeds.
More revelations affirmed the hybrid style of Ardi’s locomotion: she climbed trees, but also walked erect on the ground. Although badly damaged, Ardi’s pelvis showed muscle attachments unique to bipeds – alongside other anatomy typical of arboreal apes. As the discovery team later reported, “It is so rife with anatomical surprises that no one could have imagined it without direct fossil evidence.”
Ardi defied predictions in many ways. By the time she was discovered, molecular biology had amassed compelling evidence that humans were closely and recently related to chimpanzees (at the time scientists estimated the two lineages diverged as recently as 5 million years ago, but most now think the split was much earlier). Many scholars shared the expectation: the older the fossil, the more it would resemble a modern chimp or bonobo.
But Ardi did not knuckle walk like modern African apes – and showed no anatomical hints of descent from any such knuckle-walking ancestor. She lacked the dagger-like canine teeth of chimpanzees and her snout was less prognathous. She looked unlike anything ever seen before – what her discoverers described as “neither chimpanzee nor human.”
Ardi sparked great controversy. Some peers refused to believe that she was a member of the human family – and thus refused to accept all her disturbing implications. Others insisted she actually was more like chimp than acknowledged by the discovery team.
Over the last decade, a number of independent scholars have examined the fossils and affirmed that Ardi indeed was a hominin (formerly called hominid), a creature on our branch of the family tree after we split from the ancestors of chimps. Not every claim has won wide acceptance, but Ardi certainly forced a major rethinking of our origins. Gradually, the debate has shifted from whether to accept Ardi into the human family to how to do so.
Ardi was an inconvenient woman who did not slot easily into prevailing theory. As we go deeper into the past, our ancestors look more like apes (though not necessarily like modern apes) and the clues that link them to us become more subtle – and controversial. (The traits that ally Ardi with the human family include diamond-shaped canine teeth, bipedal features of the pelvis and foot, anatomy in the base of the skull, and more.)
Ardi represented something entirely new – a hitherto-unknown climber with an opposable toe and odd upright gait. It was not only a new species but an entirely new genus. By contrast, Lucy slotted easily into the existing genus Australopithecus because she was an older variation on a well-established anatomical theme.
As a consequence, Lucy remains much more famous than Ardi. The discoverer of Lucy, Don Johanson, excelled at public relations, wrote popular books, starred in television documentaries, and turned his skeleton into a household name.
In contrast, the Ardi team – which included several veterans of the Lucy team – eschewed that style. They worked in isolation, took 15 years to publish their skeleton, and engaged in numerous spats with peers. The Ardi team aggressively challenged prevailing theories – particularly the notion that we evolved ancestors that looked like modern chimps or the longstanding belief that expanding African savannahs played a crucial role in human evolution. Such disagreements blinded some peers to the scientific value of the oldest family skeleton.
Both skeletons testify to the importance of fossils. Theories and analytical models are essential components of science, but hard evidence sometimes defies predictions.
Despite the hype that often comes with big discoveries, no single fossil represents the beginnings of humankind, the mother of humanity, or the missing link. Rather, they are just random relics of ancient populations that we are lucky enough to find – and probably a fraction of the past forms that have been erased by time.
In the quarter of a century since Ardi was discovered, the ranks of our family have roughly doubled and there are now more than two dozen species of hominins. This includes three species older than Ardi, the most ancient being the skull of Sahelanthropus tchadensis, at least 6 million years old from Chad. Sadly, none of these older species are complete enough to include a skeleton.
Fortunately, Ethiopia has yielded more skeletons of Lucy’s species. They include a child named “Selam” (Peace) and a large male who stood a head taller than Lucy named, appropriately enough, “Kadanuumuu” (Big Guy). Another surprise: a hominin with an opposable toe who lived 3.4 million years ago at the same time as Lucy’s species – revealing that at least two types coexisted in close proximity, one bipedal and another arboreal.
Meanwhile, Kenya and South Africa have produced additional discoveries – and demonstrated that our origins are far more complex than they seemed in the old days when there were fewer dots to connect.
As more branches are named, anthropologists frequently have proclaimed that our family tree is better described as a bush. But recent advances in genomics show that neither metaphor is quite right. Ancient DNA shows that different “species” – such as Neanderthals and modern Homo sapiens.
Because the branches rejoin, our family looks not like a tree or bush and more like a mesh – complex mix of populations that dispersed, adapted to local conditions, and occasionally remixed. Our ancestors, even arboreal ones, do not easily fit in trees.
New discoveries present us with a paradox: the more we learn, the more we confront what we don’t know. More than two centuries ago, the pioneering British chemist Joseph Priestley offered a wonderful metaphor for scientific progress: as the circle of light expands, so does its circumference – the frontier between the light of knowledge and the darkness of the unknown.
As Ardi and Lucy attest, we are the last survivors of a peculiar lineage and we must painstakingly reconstruct our complex history bone by bone.
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