Geography And History Shape Genetic Differences In Humans

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ScienceDaily (June 5, 2009) — New research indicates that natural selection may shape the human genome much more slowly than previously thought. Other factors -- the movements of humans within and among continents, the expansions and contractions of populations, and the vagaries of genetic chance – have heavily influenced the distribution of genetic variations in populations around the world.

The study, conducted by a team from the Howard Hughes Medical Institute, the University of Chicago, the University of California and Stanford University, is published June 5 in the open-access journal PLoS Genetics.
In recent years, geneticists have identified a handful of genes that have helped human populations adapt to new environments within just a few thousand years—a strikingly short timescale in evolutionary terms. However, the team found that for most genes, it can take at least 50,000-100,000 years for natural selection to spread favorable traits through a human population. According to their analysis, gene variants tend to be distributed throughout the world in patterns that reflect ancient population movements and other aspects of population history.
"We don't think that selection has been strong enough to completely fine-tune the adaptation of individual human populations to their local environments," says co-author Jonathan Pritchard. "In addition to selection, demographic history -- how populations have moved around -- has exerted a strong effect on the distribution of variants."
To determine whether the frequency of a particular variant resulted from natural selection, Pritchard and his colleagues compared the distribution of variants in parts of the genome that affect the structure and regulation of proteins to the distribution of variants in parts of the genome that do not affect proteins. Since these neutral parts of the genome are less likely to be affected by natural selection, they reasoned that studying variants in these regions should reflect the demographic history of populations.
The researchers found that many previously identified genetic signals of selection may have been created by historical and demographic factors rather than by selection. When the team compared closely related populations they found few large genetic differences. If the individual populations' environments were exerting strong selective pressure, such differences should have been apparent.
Selection may still be occurring in many regions of the genome, says Pritchard. But if so, it is exerting a moderate effect on many genes that together influence a biological characteristic. "We don't know enough yet about the genetics of most human traits to be able to pick out all of the relevant variation," says Pritchard. "As functional studies go forward, people will start figuring out the phenotypes that are associated with selective signals," says lead author Graham Coop. "That will be very important, because then we can figure out what selection pressures underlie these episodes of natural selection."
But even with further research, much will remain unknown about the processes that have resulted in human traits. In particular, Pritchard and Coop urge great caution in trying to link selection with complex characteristics like intelligence. "We're in the infancy of trying to understand what signals of selection are telling us," says Coop, "so it's a very long jump to attribute cultural features and group characteristics to selection."
Journal reference:
Coop G, Pickrell JK, Novembre J, Kudaravalli S, Li J, et al. The Role of Geography in Human Adaptation. PLoS Genetics, 2009; 5 (6): e1000500 DOI: 10.1371/journal.pgen.1000500
Adapted from materials provided by Public Library of Science, via EurekAlert!, a service of AAAS.

Chemists see first building blocks to life on Earth

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British scientists said on Wednesday that they had figured out key steps in the process by which life on Earth may have emerged from a seething soup of simple chemicals.

Genetic information in living organisms today is held in deoxyribonucleic acid (DNA), the famous "double helix" molecule of sugar, phosphate and a base.
But DNA is too sophisticated to have popped up in an instant, and one avenue of thought says its single-stranded cousin, ribonucleic acid, or RNA, came first.
RNA plays a key role in making proteins and, in viruses, is used to store genetic code.
It is chemically similar to DNA but is simpler and tougher in structure, and thus looks like a good candidate for Earth's first information-coding nucleic acid.
But for all its allure, the "RNA first" theory has run into practical problems.
Its three ingredients -- the base, ribose sugar and phosphate -- must have formed separately and then combined to form the molecule, according to conventional thinking.
Critics, though, say that RNA, while somewhat simpler than DNA, is still a complex molecule and could not have been assembled spontaneously.
These doubters have been comforted by the failure to find any feasible chain of chemical events to explain how the three components all came together.
But a paper published in the British journal Nature by University of Manchester chemists puts forward a different explanation.
The team, led by Professor John Sutherland, venture that an RNA-like synthesis took place through a series of chemical reactions and an important intermediate substance.
Their lab model uses starting materials and environmental conditions that are believed to have been around in early Earth and are also used in the standard "RNA first" scenario.
Their theory starts with a simple sugar called glycolaldehyde, which reacts with cyanmide (a compound of cyanide and ammonia) and phosphate to produce an intermediate compound called 2-aminooxazole.
Gentle warming from the Sun and cooling at night help purify the 2-aminooxazole, turning it into a plentiful precursor which contributes the sugar and base portions of the new ribonucleotide molecule.
The presence of phosphate and ultraviolet light from the Sun complete the synthesis.
In a commentary also published by Nature, US molecular biologist Jack Szostak hailed the research as an elegant explanation as to why the sugar and base would not have to form separately before forming the new molecule.
"It will stand for years as one of the great advances in prebiotic chemistry," the term for the study of the chemical processes that led to life on Earth, he enthused.
Opinions vary as to when the first organisms appeared on Earth.
One estimate, based on fossilised mats of bacteria found in Australia, is that this happened around 3.8 billion years ago, around 700 million years after the planet was formed.
(c) 2009 AFP

A Genetic Trigger For The Cambrian Explosion Unraveled?

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http://www.sciencedaily.com/releases/2007/08/070831180409.htm

Science Daily — A team of scientists led by young Croatian evolutionary geneticist Tomislav Domazet-Lošo from Ruder Boškovic Institute (RBI) in Zagreb, Croatia, developed a novel methodological approach in evolutionary studies. Using the method they named 'genomic phylostratigraphy', its authors shed new and unexpected light on some of the long standing macroevolutionary issues, which have been puzzling evolutionary biologists since Darwin.

The only direct method of research in evolutionary history involves analyzing the fossil remains of once living organisms, excavated in various localities throughout of the world. However, that approach often cannot provide the full evolutionary pathway of some species, as it requires uncovering of many fossils from various stages of its evolutionary history. As the fossil record is imperfect, the evolution research fundamentally hinges on luck factor in discovering the adequate paleontological sites.
However, the RBI team proposed a novel and interesting approach to bypass this obstacle. Namely, they suggested that the genome of every extant species carries the ‘snapshots’ of evolutionary epochs that species went trough. What's even more important, they also developed the method which enables evolution researchers to readily convert those individual 'snapshots’ into the full-length 'evolutionary movie' of a species.
Applying their new methodology on the fruit fly genomic data they tackled some of the most intriguing evolutionary puzzles - some of which distressed even Darwin himself. First, they demonstrated that parts of the living organism exposed to the environment – so called ‘ectoderm’ - are more prone to evolutionary changes. Further, they explained the evolutionary origin of the ‘germ layers’, the primary tissue forms that form during the first days after the conception of a new animal, and from which subsequently all other tissues are developed. Finally, they discovered the potential genetic trigger for the 'Cambrian explosion', a major global evolutionary event on the planet, when some 540 million years ago almost all animal forms known today suddenly 'appeared'.
The first public lecture on these findings will be given by dr. Domazet-Lošo on September 4th at 5. ISABS Conference in Forensic Genetics and Molecular Anthropology, held in Split, Croatia. The groundbreaking paper fully presenting the theory of genomic phylostratigraphy will appear in the November issue of 'Trends in genetics', the most established monthly journal in Genetics.
Reference: Domazet-Lošo, T. et al. (2007) 'A phylostratigraphy approach to uncover the genomic history of major adaptations in metazoan lineages'. Trends in Genetics (to appear in the November 2007 issue of the journal)
Note: This story has been adapted from a news release issued by Rudjer Boskovic Institute.

Fausto Intilla

www.oloscience.com