An international research team has discovered a huge number of fragments of the genome that are exclusive to each of the primate species, including humans.Two researchers at the Institute of Evolutionary Biology (UPF-CSIC), Tomàs Marquès-Bonet and Arcadi Navarro, have been involved in this research which helps us to understand the mechanism of human evolution and the basis of many diseases unique to our species. On Thursday, 12th February, the journal Nature is due to publish this important scientific discovery in a special issue it has prepared to mark the 200 ^th anniversary of the birth of Charles Darwin.

During the last decade, it had come to be accepted that there was a difference between humans and our closest living relatives, chimpanzees, in only 1.24% of our DNA sequences. The discovery to be published in Nature on 12th February shows that the figure of 1.24% is completely wrong and that, in fact, it may be as much as ten times greater.

The key to this discovery was a study of segmental duplications, large fragments of DNA that are repeated many times throughout the genome but which until very recently were difficult to distinguish. This difficulty meant that they were ignored while parts of the genome that were easier to individualise were studied. This led to a skewed view of genomes.

For some time now it had been suspected that the presence of duplications must be important. But it was the systematic study of the segmental duplications in the entire genome of four primate species (macaques, orangutans, chimpanzees and humans) which enabled the progress that is to be reported in Nature to be made. It thus proved possible not only to prepare the first species-specific catalogue of these regions of the genome, but also to quantify much more accurately the differences between species and understand at what point in the evolution of our lineage they appeared.

Tomàs Marquès-Bonet, researcher at the Institute of Evolutionary Biology (IBE) -a joint research centre of Universitat Pompeu Fabra and the Spanish National Research Council (CSIC)-, co-ordinated the international team which, directed by Evan E. Eichler of the University of Washington (Seattle, USA) and, in collaboration with Arcadi Navarro (Catalan Institution for Research and Advanced Studies, ICREA-IBE Barcelona), made this major scientific discovery.

Humans and primates are qualitatively different

It should be pointed out that these regions of the genome that have been identified are qualitatively different evolutionary features. The differences on which work had been done previously were changes (mutations) in the DNA sequences codifying information for genes shared by all primates. These changes were equivalent to having different editions of the same book.

The differences studied by the IBE researchers are radical differences in each species' library: they are complete books which certain organisms possess, but others do not. They are unique differences for each species.

Segmental duplications are fragments of the genome, ranging from thousands to millions of nucleotides in length, which, due to highly complex molecular mechanisms, have duplicated themselves. In other words, at certain moments in evolution, multiple copies were made that gradually inserted themselves into various places in the genome. As the duplications can be extremely large, they contain complete genes many times over. The copies of these genes, which in theory are identical, may gradually take on specialised functions (by acquiring small mutations) until they are become completely different from each other.

This is how most of the genes unique to a specific species are generated - by duplication and subsequent specialisation. All these new genes are capable of performing new functions which, naturally, will be exclusive to the species that has them.

Tomàs Marquès and Arcadi Navarro have said, "we know the duplications predispose the genome to reorganise itself, to have major structural changes (like when someone makes different constructions with the same Lego pieces), and that can result in certain diseases (such as autism, schizophrenia or mental retardation). Gene duplication is not synonymous with anomaly, but with variation and new developments. New developments which, depending on circumstances, may be favoured by natural selection or may turn out to be pathological".

A new specialisation that began six million years ago

Another important aspect of the research is that it has succeeded in dating the period during which most duplications occurred. This was 8-12 million years ago, just before the separation of the human and chimpanzee lineages (which happened about six million years ago), implying that all these genes that had just appeared have gradually acquired new characteristics over the six million years humans and chimpanzees have evolved separately.

The researchers Tomàs Marquès and Arcadi Navarro explained that "perhaps it is thanks to this radical type of mutation that we humans succeeded in adapting to our environment. It may be that it is in this immense ocean of differences that one must look for the ‘genes of humankind' or the genes that give rise to certain diseases specific to our species. We don't know yet, but we're working on it".

The line of research that has been opened up on the basis of this work is finding out which are the regions of the genome that differ between different individuals within the same species and studying in detail various specific cases.

FIGURE. An example.

The genomes of humans, chimpanzees and gorillas differ more than was previously thought. This example shows the extreme case of a duplication that has expanded (the number of copies has increased) in the genomes of the chimpanzee and the gorilla, while it has remained stable in the human genome, where it is represented by a single copy. The green arrows and the circle mark the position of the ancestral copy. The red arrows indicate where the new copies created in the chimpanzee's and the gorilla's genomes are located.

Reference article:

Tomàs Marquès-Bonet; Jeffrey M. Kidd; Mario Ventura; Tina A. Graves; Ze Cheng; LaDeana W. Hillier; Zhaoshi Jiang; Carl Baker; Ray Malfavon-Borja; Lucinda A. Fulton; Can Alkan; Gozde Aksay; Santhosh Girirajan; Priscillia Siswara; Lin Chen; Maria Francesca Cardone; Arcadi Navarro; Elaine R. Mardis; Richard K. Wilson; Evan E. Eichler. "A burst of segmental duplications in the genome of the African great ape ancestor", Nature (12 February 2009).