How Did Humans Develop?

Today, it's a widely accepted fact that humans originated in Africa. But less than a century ago, anthropologists assumed that Eurasia was the birthplace of humanity. And scientists held onto that mistaken belief until one man took a stand that rewrote history.

In 1923, Raymond Dart arrived at the University of the Witwatersrand in Johannesburg to take a post as the head of its anatomy department. The 30-year-old Australian physician, an expert in neuroanatomy, was disappointed to learn that the university did not own a reference collection of bones and fossils. He set out to amass one, offering his students a prize for the most interesting bones they could find. His lone female student, Josephine Salmons, soon presented him with a South African fossil that would lead to the discovery of a lifetime. The fossil, a baboon cranium, sparked Dart's interest, since only two primate fossils had been found in sub-Saharan Africa until then. Salmons had found the fossil at the home of the director of the Northern Lime Company at Taung, a mining site in South Africa. Dart asked the manager of the site to alert him to any fossils that his miners unearthed in the future.

One Saturday in 1924, two boxes of rocks from Taung were deposited at Dart's door. In the second box, he came across an exciting find: an endocast, the fossilized imprint of an animal brain. To his astonishment, the endocast showed a brain larger than that of a chimpanzee but smaller than those of known human ancestors.

Digging through the box, Dart located the matching limestone rock that he knew might contain the face to match the brain. His thoughts turned to Charles Darwin, who, in his 1871 book The Descent of Man, predicted that humans' earliest apelike ancestors would be discovered in Africa because our closest ape cousins—chimpanzees and gorillas—lived there. Darwin's prediction had been generally discredited. After all, only two ancestral human fossils had ever been found in Africa and both were relatively recent, closer to modern humans than to apes. Dart wondered whether he had stumbled upon proof of Darwin's controversial theory.

For the next 73 days, Dart scraped away at the matrix, the limestone surrounding the fossilized face. Finding his hammer and chisel too clumsy, Dart turned to his wife's knitting needles, which he had sharpened to a point. His efforts were rewarded when the rock finally parted to reveal the face of an apelike child with a full set of baby teeth and molars beginning to appear.

Despite the primitive face, the child's skull, teeth and jaw clearly resembled those of humans, and the position of the opening at the back of the skull, where the spinal cord meets the brain, indicated that it walked upright, known as bipedalism. Here, Dart realized, was an early human ancestor. In a 1925 article in the journal Nature, Dark introduced the new species, which he named Australopithecus africanus, meaning "southern ape from Africa." The fossil became known as the Taung child.

Dart's discovery set off a firestorm. The leading European scientists, who preferred to believe that humans originated closer to home, were skeptical of his find. An abundance of physical evidence, including fossils and cave paintings, seemed to point to Eurasia as the cradle of humankind, and a British fossil known as Piltdown man was the most convincing evidence that Dart was incorrect. Charles Dawson, an English soldier and amateur antiquarian, discovered the bones in a gravel pit in Piltdown Common in Sussex between 1910 and 1912. Not only was Piltdown man conveniently European, but it looked like what scientists expected human ancestors to look like. It had a simian jaw and teeth but a modern-human-size brain. At the time, scientists assumed that our large brain evolved before other human traits. Piltdown man was, therefore, considered irreconcilable with the Taung child, with its humanlike jaw and teeth but small brain.

In Europe, scientists and the press widely dismissed the Taung child and ridiculed Dart. Most paleontologists believed the fossil was of a young ape, most likely a chimpanzee. Dart traveled to England in 1931 to show the fossil at scientific conferences, but he did not gain many supporters. He did not touch the fossil again for years.

Dart was not the first researcher whose fossil was unfairly rejected by European scientists. In 1893, Dutch physician Eugene Dubois announced the discovery of a set of well-preserved fossils from the Indonesian island of Java. Dubois felt that the three bones—a skullcap, molar and femur—belonged to a human ancestor that walked upright. He called the species Pithecanthropus erectus, or upright ape-man, and the fossil became known as Java man.

When he returned to Europe, Dubois was surprised to be met with resistance. Like the Taung child, Java man's skullcap indicated a relatively small brain. The human brain, scientists said, must have reached modern proportions before the creature could have been capable of walking upright. Dubois's colleagues immediately dismissed the fossil as a large gibbon or a deformed modern human.

In 1940, Dubois died without receiving the credit he was due. Java man has since been reclassified as Homo erectus, the enormously successful species that thrived for 1.5 million years starting around 1.7 million years ago and that may have directly preceded Homo sapiens, or modern humans. As the first H. erectus specimen ever found, the 900,000-year-old Java man is among the most important human-lineage, or hominin, fossils in the world.

Fortunately for Raymond Dart, thanks to some important allies, recognition came within his lifetime. Although European scientists rejected his findings, his article in Nature inspired a colleague, the Scottish scientist Robert Broom, to prove that Dart's fossil was indeed an early human and that our earliest ancestors hailed from Africa. At the age of 70, Broom, a respected paleontologist and curator of vertebrate fossils at the Transvaal Museum in Pretoria, set out to find an adult Australopithecus in Africa—something to silence Dart's critics once and for all.

Broom found such evidence in 1936, when he began acquiring fossils from caves at Sterkfontein, a site just south of Johannesburg. His early finds included the skull of an adult australopithecine. After World War II, Broom found the limb bones of a different hominin species, Paranthropus robustus, at a nearby site. With the two fossils in hand, Broom was able to confirm that these early ancestors walked upright.

These finds infected Dart's students with enthusiasm, and one of them convinced him to go fossil-hunting in the 1940s. In the caves at Makapansgat, 150 miles north of Sterkfontein, Dart and his colleagues found several australopithecines. But Dart wasn't truly vindicated until 1947, when Broom found the fantastically well-preserved skull of an adult female A. africanus at Sterkfontein. The evidence could no longer be denied: The Taung child was a hominin, not an ape. That same year, the British Association for the Advancement of Science passed a resolution stating that Broom's discoveries provided "a vindication of the general view put forward by Professor Raymond Dart in his report of the first Australopithecus skull found in 1924."

By 1953, Piltdown man had become a clear anomaly in the growing hominin fossil record. That year, scientists at Oxford and the British Museum revealed that the English fossil had, in fact, been a hoax. The culprit—whose identity remains a mystery—pieced together the fossil from a 600-year-old human cranium, an orangutan jaw and teeth, and perhaps a chimp tooth. The bones had been chemically stained and the teeth worn down to mimic human usage. With Piltdown man out of the way, the African-origin theory gained widespread acceptance.

The final confirmation came in 1959, when British archaeologist Mary Leakey made an unexpected discovery in East Africa's Rift Valley. Out walking her dogs one morning in Tanzania's Olduvai Gorge, Leakey noticed a bone sticking out of the sand. When she brushed the dirt off, she was staring into the dark eye sockets of a near-complete skull of a previously unknown species, one that her husband, archaeologist Louis Leakey, later named Zinjanthropus boisei. The fossil, with its huge cheekbones and sharply crested skull, was an amazing find. And like the Taung child, it had a brain smaller than that of modern humans but larger than modern apes.

In 1961, geologists at the University of California at Berkeley used a new technique called potassium-argon dating to accurately place the species, whose scientific name had been changed to Paranthropus boisei, in history. According to this groundbreaking method, which involves dating the rocks surrounding the specimen, the skull was 1.75 million years old—four times as old as previously thought and 750,000 years older than the accepted time frame for all of human history.

Nutcracker man, as the skull was known, was the conclusive fossil evidence that the earliest human ancestors lived in Africa, despite the fact that they didn't look like what scientists expected. Researchers streamed to East Africa in a fossil rush aimed at finding our earliest ancestors. Over the next few decades, a series of spectacular discoveries in East Africa pushed human origins deeper into the past.

In 1974, researchers in the Afar region of Ethiopia discovered "Lucy," an astonishingly complete three-million-year-old A. africanus skeleton. Lucy could clearly walk upright and was considered our oldest ancestor for a time. More recently, fossil finds have pushed the date of our earliest ancestor back further, to between five million and six million years ago, and in 2002, French paleontologist Michel Brunet announced the discovery of the oldest species yet, the six- to seven-million-year-old Sahelanthropus tchadensis. Brunet found Toumai, as the oldest hominin fossil ever found has come to be known, in the Djurab Desert of northern Chad.

In the 1960s, when potassium-argon dating was developed, paleontology began to go high-tech. For the first time, fossils older than 50,000 years could be dated. Genetic technology also came into play that decade, and today it is revolutionizing the study of human origins.

Vincent Sarich and Allan Wilson of the University of California at Berkeley were pioneers in the application of genetics to paleoanthropology in the 1960s. Their comparisons of chimpanzee and human DNA showed that the chimp lineage split from our common ancestor five million years ago. At the time, the findings caused an uproar. Anthropologists, who believed that humans and chimps diverged 15 million years ago, rejected the theory.

Recent genetic studies, however, support Sarich and Wilson's early work. The complete human genome was published in 2004, and the chimp genome followed in 2005. That year, scientists at Arizona State University and Pennsylvania State University compared modern human mitochondrial, or maternal, DNA with chimpanzee, macaque and mouse DNA to determine the point at which each lineage diverged from our common ancestor. "Though we will never know the exact date of the split, we can estimate that date using differences in their DNA," explains Blair Hedges, an evolutionary biologist at Penn State. These differences, or mutations, are assumed to occur at a constant rate, which can be used to estimate how much time has passed since lineages diverged. This method, called the molecular clock, indicated that the human and chimp lineages split five million to seven million years ago, although more fossil-based research is needed to confirm that idea.

Advances in genetic testing are also answering long-standing questions about Homo neanderthalensis, better known as Neanderthals. Modern humans and Neanderthals coexisted 30,000 years ago, and many anthropologists have wondered whether the two species ever mated. In 1997, scientists at the University of Munich in Germany extracted the first Neanderthal DNA from an upper-arm-bone fossil found in 1856. Several years later, researchers at Lawrence Berkeley National Laboratory and the Joint Genome Institute, operated by the University of California, partially sequenced the Neanderthal genome, which allowed scientists to look for Neanderthal-specific sequences in modern human mitochondrial DNA. The results showed that Neanderthals were 99.5 percent genetically identical to modern humans. "But the Neanderthal sequences for this gene do not show any equivalent to current modern human populations," says Ludovic

Orlando, an assistant professor at the École Normale Supérieure in France, who has also worked with Neanderthal DNA. Although it's possible that Neanderthal sequences could have been weeded out of our genome over time, the research strongly suggests that the two species never mated.

Some of the most exciting molecular research in anthropology today focuses on ancient human migration. African populations possess the greatest diversity of genetic mutations, which supports fossil evidence that the human lineage began there. But what happened next? Scientists are looking at genetic mutations in isolated populations around the world to answer that question. In the coming years, this data, coupled with the fossil record—Dart's Taung child included—will help confirm what routes modern humans took out of Africa 100,000 years ago.