Modern humans left the home continent in at least two waves

Apr 30, 2014 by Daniel Zadik, The Conversation
Modern Papuans have descended from beachcombers. Credit: cifor, CC BY

It is well established that modern humans originated in Africa, before moving out to inhabit rest of the planet. They first spread into Asia and Europe via the Arabian Peninsula, and those in the Far East eventually reached America and the Pacific islands.

However, this simple picture does not explain several groups found across Asia and Oceania. Now, by looking at genetic and archaeological data, researchers think they might have found the answer, confirming theories that humans migrated out of Africa more than once.

Across Asia, people are usually similar in appearance to those around them. However, there are scattered populations on islands and in other isolated areas that look quite distinct. These people are sometimes collectively called Negritos (while this may sound archaic, it is the accepted scientific term). Along with Papuans, Melanesians and aboriginal Australians, they are generally much darker-skinned and curlier-haired than their neighbours.

One explanation is offered by the "beachcomber" theory. The first that settled in Arabia were probably east African fisher-folk who crossed the Red Sea in boats. In this new land they stuck to their coastal lifestyle, rather than head inland for a whole new set of challenges. As their numbers increased, with the sea as a reliable food source and with boats for mobility, they could spread very quickly along the coast of South Asia, crossing inlets and reaching islands, until they eventually found and populated Australia. Later, inland Asian lifestles could become established and support much larger populations, which could spread south, replacing or absorbing our beachcombers in all but the most isolated locations.

This neat hypothesis seemed to have the problem solved until genetic studies were done, which grouped each Negrito population with its neighbours, rather than with other Negritos and Australasians. So why the similar appearance? Could it be that they have each separately evolved the same set of useful traits to live in a similar hot, coastal environment, in which case why have their neighbours not done the same?

A new study published in the Proceedings of the National Academy of Sciences, tackled this conflicting evidence. Scientists used mathematical modelling to explain the genetics as well as the skull shapes observed across many Asian and Australasian populations. This involved testing several alternate histories to see which one is best able to explain the modern situation. Each model must be simple enough to understand, and between them, they must cover the likely possibilities.

The models tested could be described as:

  1. A population travelled eastwards inland and spread south from there
  2. A population travelled along the beachcomber route and then spread north
  3. A population took each route without interbreeding
  4. A population took each route, they met and interbred

The reality is, of course, much more complicated, with millions of individuals living, travelling and breeding with no idea of an overall pattern. To this day, people leave Africa (and settle there), or in other words, there have been many, many out-of-Africa migrations. Nevertheless, identifying the model that best explains your observations can give you a good approximation of the most significant truths.

The four models charted. Credit: Reyes-Centeno/PNAS 2014

The study found that the fourth model best explained both the genetic data and the skulls for the Negrito . This means that there were at least two significant out-of-Africa migrations contributing to today's populations – one taking a coastal route and the other an inland route.

Negrito populations appear to have a mixture of beachcomber and inland ancestry. Australians, Melananesians and Papuans seem to descend from beachcombers alone. While other Asian populations – including Dravidian speakers, the majority of south Indians, also sometimes suggested as descendants of beachcombers – appeared to descend predominantly from the inlanders.

The timing, however, was crucial. If the Australasians had no inlander ancestry, they must have passed through Asia before the inlanders appeared. And indeed it appears they did.

A timescale was fitted to the model, using both archaeological evidence and the accumulation of genetics differences between modern populations. This suggests that, not only did the beachcombers arrive in Australia around 50,000 years ago (when the inland route was just starting out), but that they left Africa around 130,000 years ago.

This is much earlier than most previous estimates, and relatively soon after the first evidence of modern humans (around 200,000 years ago). Intriguingly, from 135,000 years ago, East Africa was struck by a series of "megadroughts". Perhaps it were these that triggered beachcombers to look for pastures new.

Explore further: Paleoanthropologists use models to show humans may have left Africa earlier than thought

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jerryd
not rated yet Apr 30, 2014
I think this whole idea of races and various so called earlier humans are all just different variations of the same race, just those who live in the tropics got darker because that protects them from Vitamin D poisoning, skin cancer, etc.

And those living far north got lighter to get more Vitamin D to live, made in the skin by sunlight. thus why we tan as we get more sun, to protect us.

Anytime you have a isolated populations variations go up but are they really different races, subspecies, or just normal variation of a far flung species with local adaptions?

Race is a human construct mostly to pit one group against another, etc for power, greed. Since they obviously breed well with each other we are the same.

As is the assumption we are smarter than they were/are. Fact is back then they would whip our bu--s. They might have to be smarter, remember better than we is as least as likely as the reverse.
JVK
1 / 5 (3) Apr 30, 2014
Large Numbers of Novel miRNAs Originate from DNA Transposons and Are Coincident with a Large Species Radiation in Bats http://mbe.oxford...abstract

A Cluster of Olfactory Receptor Genes Linked to Frugivory in Bats
http://mbe.oxford...abstract

Taken together we now see that the morphological and behavioral differences in species from microbes to man are due to ecological variation and nutrient-dependent pheromone-controlled ecological adaptations. The ecological adaptations begin with the experience-dependent de novo Creation of olfactory receptor genes in mammals, like bats and humans, but also in other vertebrates, like birds, and invertebrates, like bees.

Thus, what we've learned about the birds and the bees links nutrient-dependent pheromone-controlled ecological adaptations to frugivory in bats and humans via conserved molecular mechanisms.
Drou
1 / 5 (2) May 01, 2014
I think the 'Out of Africa' origins of humanity is absolute bunkum. Everything points to the evolution of homo sapiens in S.E. Asia. Humans were already in South America 40 to 50 000 years ago likewise in Australia. Homo sapiens were in EurAsia 300 000 years ago. The problem here is stupidity! There were many versions of homo sapiens each evolving in isolated communities in various parts of the planet, a vastly different planet to that exists today. Early humans crossed INTO Africa some 200 000 years ago and some migrated back out of Africa some 60 000 years later. The Africans we are speaking about WERE NOT NEGROID! They were the people related to the southern African Bushman and not the negroid 'Bantu' people. Humans disseminated into so many different types from 3 meter tall giants to pigmies. They adapted to all shades of skin colour and became so diverse and unique that many could no longer breed with their ancient ancestors. Nearly all of these hybrid humans were eliminated.