Africa: ¿cuna de la humanidad?

Is Africa the cradle of humanity?

by Robert Lamb

Publicado en 2007 en http://science.howstuffworks.com/life/evolution/africa-cradle-humanity1.htm

A Masai warrior surveys the landscape of the Lewa Wildlife Conservancy in Kenya, one of the oldest areas of human occupation. Joe Sohm/Digital Vision/Getty Images

Out of Africa Theory

If you look hard enough, you'll find any number of theories about just where human beings originated -- and who knows? Maybe we really were banished from a Mesopotamian Garden of Eden, or we really did stumble out of a crashed spaceship. The prevailing theory among scientists, however, is that the first Homo sapiensevolved in Africa and, between 56,000 and 200,000 years ago, migrated into other lands.

Some anthropologists actually refer to this theory as the out of Africa II theory, as it involves a previous African exodus by tribes of Homo erectus, followed by the scattering of H. sapiens that eventually became human civilization. This wave of migration steadily washed over the entire world for tens of thousands of years. The pockets of primitive hominids, such as Neanderthals, eventually disappeared. Perhaps they weren't able to compete with H. sapiens for limited resources or weren't as capable of adapting to survive the constant rigors of a prehistoric Earth.

Studies continue to back up the out of Africa theory. In studying the genetic diversity and skull shapes of 53 human populations from around the world, a team of Cambridge researchers discovered interesting proof to support the idea. They found that the farther the population was from Africa, the less varied its genetic makeup. The reason being that, as humans spread out from the cradle of civilization, their population sizes dropped. That also meant there was less genetic diversity to go around.

Think of it in terms of a group of friends getting together for a night out after work on Friday. The later it gets, the more people drop out. By 1 a.m., the group of revelers is much smaller, encompassing far less diversity.

So it seems that, yes, everything began in Africa, a continent still swimming in the genetic origins of all humanity.

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Africa cradle of humankind, study shows

fuente: http://www.theguardian.com/science/2007/jul/19/genetics.uknews

Ian Sample

The Guardian, Thursday 19 July 2007

Modern humans took their first steps to colonise the world from a single region of sub-Saharan Africa, according to British scientists.

Many scientists have long thought humankind emerged from the continent some 56,000 years ago, before spreading northwards and establishing Stone Age cultures across Asia, Europe and Australia. But some have argued that separate populations of Homo sapiens arose in different regions, before independently taking over fresh territories as their populations grew.

Andrea Manica and Bill Amos at Cambridge University used a combination of genetics and skull measurements to show modern humans could not have emerged in different places, but instead came from one region of southern central Africa.

The researchers examined genetic diversity in 53 human populations across the world and found that the further those populations were from Africa the less varied their genetic make-up. The smooth fall in genetic diversity away from Africa suggests smaller groups of humans spread out from a single area.

"It's thought that Africa's population was large but for some reason people started moving out of Africa and spreading out to colonise the world. As they did so, their population sizes went down, and because of that, they started losing genetic variability," said Dr Amos.

The researchers then looked at variations in the size and shape of 6,000 skulls around the world. Again, they found that those from populations most distant from Africa showed the least variety. "As genetic variability was lost when humans spread out, the variability in their appearance also declined," Dr Amos said.

The study appears in Nature.

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NdE:

Algunos gráficos pueden ejemplicar mejor las migraciones.

En el primer gráfico se representan las migraciones del modelo Out of Africa.

En ROJO: el Homo Sapiens

En AMARILLO los Neanderthal

En VERDE: los Primeros homínidos.

Migración analizada por el DNA mitocondrial.

Fuente: internet.

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TEORÍAS SOBRE EL ORIGEN Y DISPERSIÓN DE LOS HUMANOS ANATÓMICAMENTE MODERNOS

Texto de Natalia Acevedo y Javier Rosique basado en un resumen del Capítulo 14 de Boyd R. y Silk J.B. (2001) "Cómo evolucionaron los humanos?". Ariel. Barcelona.
http://aprendeenlinea.udea.edu.co/lms/moodle/mod/resource/view.php?inpopup=true&id=54767

La mayoría de los paleoantropólogos están de acuerdo en que un cambio dramático en la morfología de los homínidos tuvo lugar durante el último período glacial. Cerca de hace 100.000 años, el mundo estaba habitado por un conjunto de homínidos morfológicamente heterogéneos: neandertales en Europa, H. sapiens arcaicos menos robustos en el este Asia y humanos un poco más modernos en el Oriente Próximo y quizás en África. Hace cerca de 30.000 años, la mayor parte de esta diversidad había desaparecido y humanos anatómicamente modernos ocupaban todo el Viejo Mundo.

La discusión del origen de los humanos modernos se formula a veces en términos de dos hipótesis enfrentadas:

1. El modelo de reemplazamiento: dice que todos los genes necesarios para el fenotipo humano moderno fueron reunidos en África, se dispersaron cuando las gentes que llevaban estos genes migraron fuera de África y reemplazaron las poblaciones locales. Este modelo se denomina a veces hipótesis Out of Africa –salida de África–.

2. El modelo multirregional: afirma que los genes que crearon el fenotipo humano moderno surgieron en todas las áreas habitadas del mundo y se mezclaron cuando las gentes de las diferentes regiones se cruzaron entre sí, intercambiando de esta manera su material genético.

Aunque estos dos modelos dominan el debate sobre el origen de los humanos modernos, existen otros modelos intermedios:

1. Las poblaciones de homínidos en Asia y África estaban conectadas por flujo genético, pero los neandertales estarían aislados en Europa; cuando los humanos modernos entraron en Europa reemplazaron a los neandertales con poco flujo genético.

2. La morfología anatómicamente moderna evolucionó en África y se dispersó a lo largo del mundo al tiempo que las poblaciones modernas salían de África. Sin embargo, hubo un extenso flujo genético entre esta población en expansión y las poblaciones existentes de H. sapiens arcaico en cada región, como consecuencia, algunos de los de los genes de neandertales y H. sapiens arcaico podrían existir todavía en poblaciones modernas.

3. Los humanos anatómicamente modernos evolucionaron en África y sus genes se difundieron al resto del mundo por medio del flujo genético, no por la migración de los humanos anatómicamente modernos y el reemplazamiento de los grupos locales.

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