On 24 November 1974, the US anthropologist Donald Johanson was scrabbling through a ravine at Hadar in the Afar region of Ethiopia with his research student, Tom Gray. The pair were looking for fossilised animal bones in the surrounding silt and ash when Johanson spotted a tiny fragment of arm bone – and realised it belonged to a human-like creature. “We looked up the slope,” Johanson later recalled. “There, incredibly, lay a multitude of bone fragments – a nearly complete lower jaw, a thighbone, ribs, vertebrae, and more! Tom and I yelled, hugged each other, and danced, mad as any Englishman in the midday sun!” Johanson and Gray drove back to their camp in jubilation, their Land Rover horn blaring. Beer was cooled in the Awash river and barbecued goat was served to celebrate their discovery – which, by any account, was a sensational one. A total of 47 bones from a single, ancient hominin (the term used to define humans and all our extinct bipedal relatives) were ultimately uncovered by Johanson and Gray at the site. The fragments they collected amounted to about 40% of a complete skeleton, and subsequent dating has shown that these remains are around 3.2m years old. At the time, it was the oldest human-like being that had ever been unearthed by fossil hunters, and she was given the name Lucy. Fifty years on, Johanson and Gray’s discovery remains one of the most notable breakthroughs ever made in the field of human palaeontology. From the pelvis, scientists concluded it belonged to a female, while her short legs suggested she had only been about four foot tall. This discovery was followed up with other, similar finds, some in Ethiopia and some in Tanzania, and in 1978, Johanson – working with a colleague, Tim White – announced that these bones, including Lucy’s, had all come from a single, previously unknown hominin species which they named Australopithecus afarensis: the Southern Ape from Afar. Johanson and White placed afarensis at the base of a tree of ancestry that led to more recent species, such as Homo erectus and later the Neanderthals and Homo sapiens. From this perspective, Lucy was the mother of humanity. And although subsequent research and other fossil finds have led to some revisions of Lucy’s elevated status, the very fact she walked upright despite her small brain was – on its own – a discovery of considerable importance, says palaeoanthropologist Chris Stringer of the Natural History Museum, London. “Human beings have three key attributes: our ability to walk upright, our capacity to make tools, and our large brains,” says Stringer. “But a crucial question is: which of these features arrived first in our evolution? What was the first step that led our ancestors to move down a road that ultimately led to the appearance of Homo sapiens?” In The Descent of Man, Darwin argued that the three human features – bipedalism, tool-making and large brains – evolved in concert, a development in one stimulating the others to evolve further. On that basis, brain enlargement would be part of human evolution from its inception. Then came the discovery of Lucy. “Lucy showed that this idea was simply not true,” says Stringer. “Her skeleton showed our ancestors walked on two feet long before their brains got big.” This point is backed by Zeresenay Alemseged, a palaeoanthropologist at Chicago University. “Lucy showed that a big brain was not the sine qua non of being a member of the human lineage,” he says. It is an intriguing observation, one that raises key questions. Why did our ancestors adopt a bipedal gait in the first place? What evolutionary advantages did they acquire in getting up on two feet? Many answers have been proposed over the years. Walking on two feet, apemen would have had arms free to pick fruits from low-lying branches and could also carry food and babies. Standing upright, they would have appeared larger and more intimidating, while reducing the level of the harsh African sunlight beating down on their backs. These are all thought-provoking suggestions, though the most probable reason was more prosaic, argues Alemseged. “When you walk on two legs, as opposed to four, you save energy. It is as simple as that. You use up fewer calories – and remember, our early ancestors were not struggling to lose weight as we do today. They needed to get all the energy they could get and exploit it with maximum efficiency. Walking on two feet helped them do that.” Humans pay for that transition to an upright gait today – in terms of back pain and other skeletal problems that arise in later life. On the other hand, we have reaped the benefits in terms of the expansion of our brains that followed, eventually, in the wake of our adoption of bipedalism. Lucy’s discovery placed afarensis at the heart of the story of human evolution. However, since her presence was first revealed in Hadar many fossils of other, even older hominin species have been found. These include Australopithecus anamensis, which – four million years ago – ambled across terrain that lies in Kenya and Ethiopia today, and Ardipithecus ramidus, which lived around 4.5m years ago in a similar patch of Africa. Crucially, these early apemen also have anatomies that suggest they were bipedal. So, could one of these species – and not afarensis – have been the true originator of the lineage that led to Homo sapiens? Lucy’s kin could merely have been a side branch of that family tree, and not a direct link to modern humans. In other words, was Lucy merely a great aunt of humanity, not its mother? Some scientists believe this could be the case. However, Alemseged has his doubts. “These earlier hominins probably walked upright for some of the time, but many were probably living in trees for most of their lives. In contrast, Lucy and her afarensis kin were spending a great deal of time walking upright. They were pivotal in the transformation of our genus into one that became committed to an upright stance.” With Lucy, our lineage reached the stage where walking upright became commonplace. We became obligate bipedal animals, the defining feature of the genus that eventually produced Homo sapiens. Alemseged’s own contribution to this field was his discovery, on 10 December 2000, of Selam, the almost complete fossil skull and parts of the skeleton of a child of Australopithecus afarensis. It is sometimes referred to as “Dikika child” or “Lucy’s child”, though this latter attribution is a misnomer, given that the skull has been dated as being 3.3m years old and is therefore more than 100,000 years older than Lucy. “We have now found afarensis in Tanzania, Chad, Kenya and Ethiopia, and we know Lucy and her kin must have lived in these parts of Africa for close to a million years,” adds Alemseged. “That antiquity and extensive geographical spread convince me that it is the most likely candidate to have given rise to the many species of the Homo genus and ultimately to our own species, Homo sapiens.” Lucy’s remains are now housed at the National Museum of Ethiopia in Addis Ababa, where Alemseged – who was born in Ethiopia – made headlines in 2015 when he was on hand to show Lucy to Barack Obama during the president’s state visit. She is the precursor of all humans today, he told Obama. “Every single person, even Donald Trump.” Other scientists are more cautious about Lucy’s exact relation to humans today. “The problem is that we have only two areas from which we have good fossil evidence of hominin evolution: in the Rift Valley areas of Kenya, Tanzania and Ethiopia; and in South Africa,” Stringer points out. “In the former, there are lakes, rivers and sediments in which it is relatively easy to find fossils, while in South Africa, there are lots of caves where early hominins became fossilised. That gives you a very biased picture of hominin evolution in Africa. We don’t know what happened elsewhere in the continent,” Stringer adds. “It is a bit like the drunk man who is searching at night for keys that he has dropped and only looks where there is street lighting – because these are the only places he can see. At present, there is a shortage of places to find [fossil remains in Africa] and of places where people have actually looked, and that limits the evidence we can gather about how, exactly, the human lineage evolved millions of years ago.” Nevertheless, it is clear that Lucy has been able to play a major part in developing our understanding of our own species – though her naming was rather haphazard, as Johanson admitted in recollections of the heady days that followed her discovery in Hadar. “Surely such a noble little fossil lady deserved a name, we all thought, and as we sat around one evening listening to Beatles songs, someone said: ‘Why don’t we call her after Lucy? You know, after Lucy in the Sky With Diamonds.’ So she became Lucy.” However, it might, quite easily, have been a very different name, as Caitlin Schrein has pointed out in Nature. The Beatles song had been recorded seven years earlier. And, if Johanson and his colleagues had been more up to date in their choice of pop music, or had a better availability of records, they would probably have been playing more contemporary tracks. Songs might even have included some of the hits of 1974 – such as Annie’s Song by John Denver or Bennie and the Jets by Elton John. Had they been listening to these tracks then the world’s most famous fossil skeleton might have a different name. The name is perhaps irrelevant, however. “The crucial point is that she was a great trailblazer for highlighting early human evolution,” says Stringer.