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The Origin of the First Humans: Evolution and Interbreeding

How Did Humans Evolve?

Thesis Proposal.

It has been a controversial argument for many years now that the origin of first human.

Thesis

The first humans came from Africa. Most experts assume that modern humans developed on the African continent approximately 200,000 years ago before spreading across the globe and becoming the dominating species we are today. Finding proof of the first Homo sapiens — our distant ancestors who separated from Neanderthals around 500,000 years ago — is difficult due to the scarcity of traces. The majority of them were discovered in East Africa.

Outlines:

The first humans emerged in Africa around two million years ago, long before the modern humans known as Homo sapiens appeared on the same continent.

There’s a lot anthropologists still don’t know about how different groups of humans interacted and mated with each other over this long stretch of prehistory. Thanks to new archaeological and genealogical research, they’re starting to fill in some of the blanks.

Homo habilis individuals chip away at rocks, sharpening them for cutting up game or scraping hides while a woman, with her child, gathers wild berries to eat and branches to make shelters.

First things first: A “human” is anyone who belongs to the genus Homo (Latin for “man”). Scientists still don’t know exactly when or how the first humans evolved, but they’ve identified a few of the oldest ones.

One of the earliest known humans is Homo habilis, or “handy man,” who lived about 2.4 million to 1.4 million years ago in Eastern and Southern Africa. Others include Homo rudolfensis, who lived  in Eastern Africa about 1.9 million to 1.8 million years ago (its name comes from its discovery in East Rudolph, Kenya); and Homo erectus, the “upright man” who ranged from Southern Africa all  the way to modern-day China and Indonesia from about 1.89 million to 110,000 years ago.

In addition to these early humans, researchers have found evidence of an unknown “superarchaic” group that separated from other humans in Africa around two million years ago. These superarchaic  humans mated with the ancestors of Neanderthals and Denisovans, according to a paper published in Science Advances in February 2020. This marks the earliest known instance of human groups mating  with each other—something we know happened a lot more later on.

After the superarchaic humans came the archaic ones: Neanderthals, Denisovans and other human groups that no longer exist.

Archaeologists have known about Neanderthals, or Homo neanderthalensis, since the 19th century, but only discovered Denisovans in 2008 (the group is so new it doesn’t have a scientific name yet). Since then, researchers have discovered Neanderthals and Denisovans not only mated with each other, they also mated with modern humans.

The First Humans

When the Max Plank Institute [for Evolutionary Anthropology] began getting nuclear DNA sequenced data from Neanderthals, then it became very clear very quickly that modern humans carried some  Neanderthal DNA,” says Alan R. Rogers, a professor of anthropology and biology at the University of Utah and lead author of the Science Advances paper. “That was a real turning point… It became  widely accepted very quickly after that.”

As a more recently-discovered group, we have far less information on Denisovans than Neanderthals. But archaeologists have found evidence that they lived and mated with Neanderthals in Siberia  for around 100,000 years. The most direct evidence of this is the recent discovery of a 13-year-old girl who lived in that cave about 90,000 years ago. DNA analysis revealed that her mother was  a Neanderthal and her father was a Denisovan.

The human lineage of Australopithecus afarensis, Homo habilis, Homo erectus, Neanderthals and Homo sapiens.

Encyclopaedia Britannica/Universal Images Group/Getty Images

Scientists are still figuring out when all this inter-group mating took place. Modern humans may have mated with Neanderthals after migrating out of Africa and into Europe and Asia around 70,000 years ago. Apparently, this was no one-night stand—research suggests there were multiple encounters between Neanderthals and modern humans.


Less is known about the Denisovans and their movements, but research suggests modern humans mated with them in Asia and Australia between 50,000 and 15,000 years ago.


Until recently, some researchers assumed people of African descent didn’t have Neanderthal ancestry because their predecessors didn’t leave Africa to meet the Neanderthals in Europe and Asia. But  in January 2020, a paper in Cell upended that narrative by reporting that modern populations across Africa also carry a significant amount of Neanderthal DNA. Researchers suggest this could be the result of modern humans migrating back into Africa over the past 20,000 years after mating with Neanderthals in Europe and Asia.

Given these types of discoveries, it may be better to think about human evolution as a “braided stream,” rather than a “classical tree of evolution,” says Andrew C. Sorensen, a postdoctoral researcher in archaeology at Leiden University in the Netherlands. Although the majority of modern humans’ DNA still comes from a group that developed in Africa (Neanderthal and Deniosovan DNA accounts for only a small percentage of our genes), new discoveries about inter-group mating have complicated our view of human evolution.

Human groups that encountered each other probably swapped more than just genes, too. Neanderthals living in modern-day France roughly 50,000 years ago knew how to start a fire, according to a 2018 Nature paper on which Sorensen was the lead author. Fire-starting is a key skill that different human groups could have passed along to each other—possibly even one that Neanderthals taught to some modern humans.

Early Humans, Neanderthals, Denisovans Mixed It Up

Publicly available L0 mitogenomes. A total of 1,019 publicly available L0 mitogenomes, spanning 26 studies, were downloaded from National Center for Biotechnology Information database between 2015 and 2017. Samples were broadly ethno-linguistically classified, per Fig. 1b as for our 198 samples, based on the samples’ reported population and/or country of origin. Reported haplogroups were confirmed or refined using HaploGrep2 based on PhyloTree Build 17.

Bayesian Skyline Plot Summary. For common haplogroups with > 100 mitogenomes, at least five sub-sampling for n=100 were performed. In contrast, for rare haplogroups with < 20 mitogenomes, BSP analyses were performed with varying Group Sizes to ensure this parameter did not dramatically impact the results. While Effective Sample Sizes (ESS) were low for the Posterior of the model, we note ESS for the Tree Likelihood and BSP generally reached acceptable levels (>500). This table shows the best BSP result for each haplogroup.

Toggle Caption Cast of Skhul 5, a 90,000-year-old skull discovered in1932 in Skhul Cave, Mount Carmel, Israel. This skull of an adult male has developed relatively modern features including a higher forehead although it still retains some archaic features including a brow ridge and slightly projecting face. This specimen and others from the Middle East are the oldest known traces of modern humans outside of Africa. They prove that Homo sapiens had started to spread out of Africa by 100,000 years ago, although it may be that these remains represent a population that did not expand beyond this region – with migrations to the rest of the world occurring later, about 60-70,000 years ago. 

Our species, Homo sapiens, has now spread to all parts of the world but it's generally believed that we originated in Africa by about 200,000 years ago. We interacted with local archaic human populations as we colonised the globe.

During the 1980s and 1990s the fossil record, while growing, was not able to clearly demonstrate whether Homo sapiens evolved from local ancestors across the globe or originated in a single region and then dispersed. DNA studies generally supported the single origin theory, but were still in their infancy. The two major and opposing opinions were known as the ‘Out of Africa’ model and the ‘Multiregional’ model.

The ‘Out of Africa’ model was the most widely favoured explanation accounting for the origins of modern humans. It suggested that modern humans originated in Africa within the last 200,000 years from a single group of ancestors. Modern humans continued to evolve in Africa and had spread to the Middle East by 100,000 years ago and possibly as early as 160,000 years ago. Modern humans only became well established elsewhere in the last 50,000 years. The different physical features now found in modern humans from different geographical areas around the world are believed to have evolved over only the last 60,000 years or so as a result of adaptations to different environments.

Human Evolution Was Messy

As modern humans spread, they replaced all other human species. Homo heidelbergensis was replaced by modern humans Africa and Europe, Homo erectus was replaced in Asia and Homo neanderthalensis was replaced in Europe. The most extreme version of this model suggested that modern humans replaced the older humans without any interbreeding. Less extreme versions allowed for some interbreeding between these populations but suggest that gene flow and mixing between these different species was extremely limited.

The ‘Out of Africa’ model has had a variety of names including:

• ‘The Garden of Eden’ hypothesis

• ‘Noah’s Ark’ hypothesis

• ‘Out of Africa 2’ hypothesis, which distinguishes the earlier and later dispersals of humans out of Africa. In this case, ‘Out of Africa 1’ refers to the initial dispersal out of Africa by Homo ergaster, whereas ‘Out of Africa 2’ refers to the later dispersal out of Africa by modern humans.

The ‘Multiregional’ model suggested that when human ancestors first left Africa nearly two million years ago, they spread out and formed regional groups of early humans across Africa, Asia and Europe. Modern humans then evolved concurrently in all these regions rather than from a single group of humans in Africa. Interbreeding between different regional populations did occur. Geographically separated populations remained genetically similar to one another through the genetic mixing that resulted from interbreeding and a single species was therefore maintained. The different physical features that are found in modern humans from different geographical areas around the world are believed to have evolved over a very long period in Africa, Asia and Europe since the time when each region became settled.

Other names for the Multiregional model:

The ‘Multiregional’ model is also known as the ‘Regional Continuity’ model.

Latest findings – including new fossils and improved DNA research and dating techniques – confirm the complexity of modern human (Homo sapiens) origins. Evidence still suggests that all modern humans are descended from an African population of Homo sapiens that spread out of Africa about 60,000 years ago but also shows that they interbred quite extensively with local archaic populations as they did so (Neanderthal and Denisovan genes are found in all living non-Africa populations) and these local populations contributed to our species’ success. So, while the general basis of the original Out of Africa model prevails, it requires extensive revision.

Evidence indicates that Neanderthal and Denisovan traits emerge in Eurasia, while Homo sapiens traits emerge in Africa. Africa and Eurasia are isolated until H. sapiens disperses and interbreeds with the other two (and possibly some other unknown archaic species). Modern humans essentially absorb and replace the fragmentary local populations. The low percentage of Neanderthal and Denisovan genes found in living humans indicates a replacement process was most likely.

Early Human Ancestors Shared Skills

What remains unclear is how ancestral modern human populations were interacting in Africa. Did Homo sapiens originate from a small local population that then spread, as some believe, or via interbreeding between multiple groups across a wide area?

New fossil discoveries suggest that modern human physical traits did not emerge as one suite but were gradual. A skull from Jebel Irhoud in Morocco dated to about 300,000 years old, controversially assigned to Homo sapiens, had a modern-looking face but an elongated, archaic-looking braincase. This suggests our globular braincase evolved later and not as part of a fully modern suite of features.

Based on this and other early Homo sapiens remains, including those from Herto and Omo Kibish in Ethiopia, the African Multiregionalism Model of H. sapiens evolution was proposed in 2018 by a group including Eleanor Scerri and Chris Stringer (who proposed the original Out of Africa model). They suggest that early Homo sapiens showed great diversity and that rather than a single origin, our species emerged from admixture among numerous populations within Africa.

While most researchers agree with the RAOWH model, there are some that propose a different theory of interactions among human species and the role these interactions had in modern human origins. This theory differs in the way it explains how the DNA of Homo sapiens mixed with local populations outside Africa. Essentially, while some H. sapiens traits originated in Africa, it was when populations spread into Eurasia and extensively interbred with Neanderthals and Denisovans, that the evolution of new modern traits occurred. Thus, this model proposes, modern human origins involve a high degree of assimilation with archaic Eurasian populations within the last 100,000 years.

How do we relate to other archaic humans given that we clearly interbred and the offspring also produced viable ancestors?

Homo sapiens share physical features and genetic characteristics that distinguish them from other humans and merit their classification as a separate species. However, some believe the fact we interbred with Neanderthals, for instance, means they should be classified as the same species under the biological species concept. However, this definition of species has limitations, particularly when it comes to defining human species.

On the biggest steps in early human evolution scientists are in agreement. The first human ancestors appeared between five million and seven million years ago, probably when some apelike creatures in Africa began to walk habitually on two legs.

They were flaking crude stone tools by 2.5 million years ago. Then some of them spread from Africa into Asia and Europe after two million years ago.

With somewhat less certainty, most scientists think that people who look like us -- anatomically modern Homo sapiens -- evolved by at least 130,000 years ago from ancestors who had remained in Africa. Their brain had reached today's size. They, too, moved out of Africa and eventually replaced nonmodern human species, notably the Neanderthals in Europe and parts of Asia, and Homo erectus, typified by Java Man and Peking Man fossils in the Far East. 

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