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HumanMigration

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New Stanford analysis provides fuller picture of human expansion from Africa

Stanford Report, October 22, 2012  |  By BJORN CAREY

A comprehensive analysis of the anthropological and genetic history of humans’ expansion out of Africa could lead to medical advances.

A new, comprehensive review of humans’ anthropological and genetic records gives the most up-to-date story of the “Out of Africa” expansion that occurred about 45,000 to 60,000 years ago.

This expansion, detailed by three Stanford geneticists, had a dramatic effect on human genetic diversity, which persists in present-day populations. As a small group of modern humans migrated out of Africa into Eurasia and the Americas, their genetic diversity was substantially reduced.

In studying these migrations, genomic projects haven’t fully taken into account the rich archaeological and anthropological data available, and vice versa. This review integrates both sides of the story and provides a foundation that could lead to better understanding of ancient humans and, possibly, genomic and medical advances.

“People are doing amazing genome sequencing, but they don’t always understand human demographic history” that can help inform an investigation, said review co-author Brenna Henn, a postdoctoral fellow in genetics at the Stanford School of Medicine who has a PhD in anthropology from Stanford. “We wanted to write this as a primer on pre-human history for people who are not anthropologists.”

This model of the Out of Africa expansion provides the framework for testing other anthropological and genetic models, Henn said, and will allow researchers to constrain various parameters on computer simulations, which will ultimately improve their accuracy.

“The basic notion is that all of these disciplines have to be considered simultaneously when thinking about movements of ancient populations,” said Marcus Feldman, a professor of biology at Stanford andthe senior author of the paper. “What we’re proposing is a story that has potential to explain any of the fossil record that subsequently becomes available, and to be able to tell what was the size of the population in that place at that time.”

The anthropological information can inform geneticists when they investigate certain genetic changes that emerge over time. For example, geneticists have found that genes that allowed humans to tolerate lactose and gluten began to emerge in populations expanding into Europe around 10,000 years ago.

The anthropological record helps explain this: It was around this time that humans embraced agriculture, including milk and wheat production. The populations that prospered – and thus those who survived to pass on these mutations – were those who embraced these unnatural food sources. This, said Feldman, is an example of how human movements drove a new form of natural selection.

Populations that expand from a small founding group can also exhibit reduced genetic diversity – known as a “bottleneck” – a classic example being the Ashkenazi Jewish population, which has a fairly large number of genetic diseases that can be attributed to its small number of founders. When this small group moved from the Rhineland to Eastern Europe, reproduction occurred mainly within the group, eventually leading to situations in which mothers and fathers were related. This meant that offspring often received the same deleterious gene from each parent and, as this process continued, ultimately resulted in a population in which certain diseases and cancers are more prevalent.

“If you know something about the demographic history of populations, you may be able to learn something about the reasons why a group today has a certain genetic abnormality – either good or bad,” Feldman said. “That’s one of the reasons why in our work we focus on the importance of migration and history of mixing in human populations. It helps you assess the kinds of things you might be looking for in a first clinical assessment. It doesn’t have the immediacy of prescribing chemotherapy – it’s a more general look at what’s the status of human variability in DNA, and how might that inform a clinician.”

The study is published in the current edition of the Proceedings of the National Academy of Sciences and was co-authored by Feldman’s longtime collaborator, population geneticist Luigi Luca Cavalli-Sforza of Stanford and the Università Vita-Salute San Raffaele in Italy.

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The great human expansion

  1. Brenna M. Henna,
  2. L. L. Cavalli-Sforzaa,1, and
  3. Marcus W. Feldmanb,2

-Author Affiliations


  1. aDepartment of Genetics, Stanford University School of Medicine, Stanford, CA 94305; and

  2. bDepartment of Biology, Stanford University, Stanford, CA 94305-5020
  1. Edited by C. Owen Lovejoy, Kent State University, Kent, OH, and approved September 25, 2012 (received for review July 19, 2012)

Abstract

Genetic and paleoanthropological evidence is in accord that today’s human population is the result of a great demic (demographic and geographic) expansion that began approximately 45,000 to 60,000 y ago in Africa and rapidly resulted in human occupation of almost all of the Earth’s habitable regions. Genomic data from contemporary humans suggest that this expansion was accompanied by a continuous loss of genetic diversity, a result of what is called the “serial founder effect.” In addition to genomic data, the serial founder effect model is now supported by the genetics of human parasites, morphology, and linguistics. This particular population history gave rise to the two defining features of genetic variation in humans: genomes from the substructured populations of Africa retain an exceptional number of unique variants, and there is a dramatic reduction in genetic diversity within populations living outside of Africa. These two patterns are relevant for medical genetic studies mapping genotypes to phenotypes and for inferring the power of natural selection in human history. It should be appreciated that the initial expansion and subsequent serial founder effect were determined by demographic and sociocultural factors associated with hunter-gatherer populations. How do we reconcile this major demic expansion with the population stability that followed for thousands years until the inventions of agriculture? We review advances in understanding the genetic diversity within Africa and the great human expansion out of Africa and offer hypotheses that can help to establish a more synthetic view of modern human evolution.

(not open access)

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Η παράθεση συνδέσμων σχετικών με την καταγωγή και εξέλιξη του ανθρώπινου είδους έχει ως μονο στόχο την ενημέρωση για την πορεία των ανακαλύψεων των απολιθωμάτων ως ιστορικών στοιχείων και δεν προωθεί καμία απολύτως άποψη ή πεποίθηση.
Η εικονικές αναπαραστάσεις με βάση τα απολιθώματα δεν θεωρούνται τεκμήρια καταγωγής του ανθρώπου.
Οι εξωτερικές ομοιότητες μεταξύ των ανθρώπινων και των ζωϊκών απολιθωμάτων δεν θα πρέπει επίσης να θεωρούνται τεκμήρια καταγωγής, αλλά συγγένειας,  καθώς ο ανθρώπινος γενετικός κώδικας ταυτίζεται σε μεγάλο ποσοστό  (και ενίοτε μεγαλύτερο)  με είδη που δεν ομοιάζουν εξωτερικά με αυτόν.
Η πλέον γνωστή, αλλά συνάμα απλοϊκή και αφελής απεικόνιση ενός οργανισμού που βγαίνει απο τη θάλασσα και καταλήγει άνθρωπος
είναι μια αναπόδεικτη προς το παρόν εικασία, οπότε επίσης δεν θα πρέπει να αποτελεί τεκμήριο. Αντιθέτως παραπλανά και λαμβάνοντας υπόψη την πολύπλευρη επίδραση που δύναται να έχει μια εικόνα στη νόησή μας, χρήζει της ιδιαίτερης προσοχής μας, όπως και κάθε παρόμοια εικόνα,  προς αποφυγή εξαγωγής απλοποιημένων συμπερασμάτων.
Η λέξεις < καταγωγή > και < εξέλιξη > οδηγούν συχνά σε ερμηνείες εξαρτημένες απο την ετυμολογία τους και από το πως εκλαμβάνονται απο τη νόησή μας, με αποτέλεσμα η πρώτη να συνεπάγεται ότι υποχρεωτικά έχω κάποιο πρόγονο και η δεύτερη ως αναπόφευκτη και κατα κάποιο τρόπο, επιβεβλημένη  μεταβολή.
Στην περίπτωση των απολιθωμάτων τα ίχνη διαχωρισμού ή συγγένειας των ειδών γίνονται διακριτά με τρόπο αντιστρόφως ανάλογο της χρονολόγησής τους και αυτό προκαλεί συχνά σύγχυση και κατάληξη σε ασαφή αντιεπιστημονικά συμπεράσματα.
Παραδόξως παρατηρούμε τέτοιου είδους συμπεράσματα να υιοθετούνται ευρέως στην εποχή μας και μάλιστα απο επιστήμονες…

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του Στ. Γ. Φραγκόπουλου, Δρ.Μηχ., Καθηγητή ΤΕΙ Αθήνας

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Physics4u’s Weblog


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New Stanford analysis provides fuller picture of human expansion from Africa.

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ResearchBlogging.org
BJORN CAREY (2012).
New Stanford analysis provides fuller picture of human expansion from Africa

Stanford University Report

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