What makes us humans?

May 8, 2007 23:46 GMT  ·  By

There is only 1.2 % genetic difference between humans and chimps, but each gene that makes the difference represents the abyss between the human and the ape. Now, researchers have discovered perhaps the most significant gene of this pool, encoding the protein called type II neuropsin, involved in human learning and memory.

The team of researchers led by Dr. Bing Su of the Chinese Academy of Sciences in Kunming, China, investigated DNA sequences in several species of apes and monkeys, compared to the human. It was known that type II neuropsin, a longer protein, does not exist in the prefrontal cortex (PFC), the site of cognition, of gibbons (lesser apes) and Old World monkeys (from which apes descended). In their research, the Chinese scientists did not found this protein in two great ape species, chimpanzee and orangutan, either.

Chimps split off from humans 4 million years ago; orangutans from the chimp-humans branch 14 million years ago, thus 4 million years is the maximum age for type II. Gene sequencing spotted a human specific mutation that induced a new splicing site and a longer protein in type I neuropsin. "Hence, the human-specific mutation is not only necessary but also sufficient in creating the novel splice form," the authors state.

But the researchers also detected a weaker effect of a different type I-specific splicing site and a drop in type I neuropsin expression in human and chimpanzee compared to the rhesus macaque, an Old World monkey from southern Asia. "This pattern suggests that before the emergence of the type II splice form in human, the weakening of the type I splicing site already existed in the common ancestor of humans and chimpanzees, implying a multi-step process that led to the dramatic change of splicing pattern in humans", the authors note.

The team detected a site in the chimp genome that induces a weakening effect on the splicing site, a feature that could have existed in the human ancestors. "It is likely that both the creation of novel splice form and the weakening of the constitutive splicing contribute to the splicing pattern changes during primate evolution, suggesting a multi-step process eventually leading to the origin of the type II form in human," the authors state.

It would be interesting to see how the extra 45 amino acids of the type II neuropsin in humans provoke changes in the protein's functions.