Many scientists once had the human story backwards. They assumed the big brain came first, and that walking upright came later. A skeleton found in Ethiopia in 1974 helped settle the order. Early members of our lineage were walking on two legs more than three million years ago, while their brains were still about the size of a chimpanzee’s.

Start with the old idea. In 1912, fossil fragments emerged from a gravel pit in southern England. The Piltdown find combined a modern-sized braincase with an ape-like jaw, fitting a belief held by many researchers: the brain led the way in human evolution, and the body followed. It was a fake, conclusively exposed in 1953. But it won acceptance for decades because it flattered a confident assumption.

As one educational science resource puts it, “in 1912 most paleoanthropologists thought the opposite” of what we now understand. The big brain, they figured, came first.

The old story: big brains first, walking second

It’s easy to see why the idea had such a grip. Our brains are the trait we often point to when separating ourselves from other animals, so it felt natural to assume the brain changed first too. Piltdown didn’t create that belief. It appeared to confirm one already there, which is probably why the crude fake retained influential defenders for so long.

The deeper evidence was scarce, fragmentary and contested. Fossils were often broken and difficult to date, leaving room for tidy theories. What the field needed was a skeleton old enough and complete enough to clarify the sequence. In 1974, it got one.

What the bones at Hadar actually showed

On November 24, 1974, Donald Johanson and Tom Gray, working at Hadar in Ethiopia, spotted a fragment of arm bone in a gully. The excavation produced a partial skeleton nicknamed Lucy. In Amharic she is called Dinkinesh.

Roughly 40 percent of a single skeleton came out of that ground, remarkable for a fossil this old. There were skull fragments, a jaw, and pieces of the arm, leg, pelvis, spine and ribs. Johanson, Tim White and Yves Coppens named her species, Australopithecus afarensis, in 1978. She stood about three and a half feet tall and weighed 60 to 65 pounds.

This is where the old story fell apart. Lucy’s braincase was small, roughly 388 cubic centimeters, compared with roughly 1,400 for a modern human. Ape-sized, in other words. But her pelvis and knee carried clear adaptations for walking on two legs, and she was around 3.18 million years old. An upright walker with a chimpanzee-sized brain, millions of years before the major brain expansion seen later in Homo.

Why walking before brains upended everything

The order was visible in one body, and it ran opposite to Piltdown. Writing on the fiftieth anniversary of the find, Johanson and Institute of Human Origins director Yohannes Haile-Selassie put it plainly: “Lucy confirmed earlier suspicions that upright walking evolved before large brains.” Note the word “confirmed.” She didn’t invent the idea single-handedly. She gave it unusually complete evidence.

Other finds helped. Fossil footprints at Laetoli in Tanzania, preserved in 3.66-million-year-old volcanic ash, recorded unequivocal bipedal walking. The best-known trails were most likely made by Australopithecus afarensis, matching what Lucy’s bones implied. The footprints and skeleton together made the sequence much harder to dispute.

What Lucy still can’t tell us

A single skeleton is a thin thing to rest a revolution on. “It can be precarious to hang such a pivotal argument on a single fossil individual,” Johanson and Haile-Selassie acknowledge.

What strengthens the argument is that Lucy is no longer alone. Her species is known from many other fossils, and the authors call Au. afarensis the “best candidate ancestor of Homo and Paranthropus.”

The tidy picture has since gotten messier. Later discoveries indicate that different early human relatives overlapped in time and place, and even different kinds of bipedal hominin may have walked at Laetoli. The straight line from ape to human now looks more like a thicket, with branches that overlap and dead-end. We don’t know exactly where Lucy sits in it. That isn’t a failure of the science but the science working, swapping one confident answer for better questions.

Lucy didn’t hand us the whole story. She helped permanently rule out the brain-first version that Piltdown had reinforced for decades. Three and a half feet tall, an ape-sized head, hips made for walking, and after fifty years of digging we still can’t say precisely who her descendants were. She left us with a better ignorance.