Early Development of Modern Humans

Early development of modern humans (known as Homo sapiens sapiens) has been an area of intense research for almost 100 years. In his book “Descent of Man” published in 1871, Charles Darwin first hypothesized that modern humans may have had a single origin. When archaeologists eventually discovered and began categorizing early human remains in Africa in the early and mid-20th century, the debate changed from one of sheer speculation to a more evidence-based movement. From the 1920’s through the 1980’s, archaeologists primarily used anatomic changes in skeletal fossils to slowly and progressively developed a human “family tree” of sorts. Though this type of archaeological research has been helpful in retracing mankind’s history, it lacked scientific certainty due to its reliance on the interpretation of recovered artifacts such as tools, art, and human remains. Consensus in many important areas had been lacking. With the advent of accurate and inexpensive DNA testing beginning in the 1990’s, this research field has exploded. DNA analysis of ethnic populations and ancient remains has led to dramatic new findings, resulting in the development of timelines, migration maps, and lineages showing connections between early human populations at a confidence level that many thought would be impossible just two decades ago.

Of these findings is Pääbo’s sequencing of the Neanderthal genome. Pääbo’s group published a study that turned previous perspectives on the modern human-Neanderthal relationship upside down. The team used samples from the Vindija cave in Croatia, preparing 9 DNA extracts from 3 bones to compare to human genome sequences. Modern human sequences came from 5 geographically diverse sources: one San from Southern Africa, one Yoruba from West Africa, one Papua New Guinean, one Han Chinese, and one French from Western Europe. The results gave completely new perspectives on old unanswered questions. One of these long standing questions regards the actual time the Neanderthal and modern human populations diverged. Their research shows that Neanderthals and modern humans split between 270,000 and 440,000 years ago, which is compatible with other paleontological and archeological predictions. It was also found that Neanderthals shared more derived alleles with non-African modern humans than Africans and that no variation in the derived alleles was apparent when comparing the Neanderthals to individuals within these two subsets. From this, it was determined that Neanderthals contributed 1% to 4% to the non-African modern human genome compared to a negligible contribution to African humans, a drastically different result from those reported by all previous mtDNA studies. This implies a great deal about the expansion of modern humans out of Africa. First, it can be inferred that a large group of modern humans separated from their forbears and migrated out of Africa. The fact that Neanderthals share the same amount of derived alleles with all non-Africans implies that the interbreeding between the migrating homo sapiens sapiens and the Neanderthals occurred before they (modern humans) inhabited other parts of the globe. This admixture then was most likely to have occurred somewhere in the Middle East.

Pääbo’s team’s sequencing of around 70% of the Neanderthal genome illustrates an entirely new picture of the early human migration patterns: a picture that archeological studies alone couldn’t have hoped to predict. Neanderthals migrated out of Africa around and split with the – what would become – modern humans around 400,000 years ago. They continued their migration north into and eventually inhabited most of the Middle East and Europe. Modern humans first successfully arose from Africa around 60,000 years ago. As they began to move across the Arabian Peninsula and made their way into the Middle East, they encountered Neanderthals and subsequent gene flow transpired. From here, the bulk of the modern humans began making their way eastward into Asia and eventually across the Bering Strait into the Americas while others continued north into Europe. For whatever reason, not many Neanderthals followed the modern humans that continued eastward (and none made it into the Americas), nor did they mate significantly with those who traveled into Europe as Pääbo’s discovery implies. By 30,000 years ago, the Neanderthals had completely vanished, and the modern humans emerged, becoming the last hominid species to occupy the planet: and we are still the only hominids around today.