A new evolutionary theory explains how critically small populations of early humans survived, despite an increased chance of hereditary disabilities being passed to offspring.
     Dr Isabelle Winder, from the Department of Archaeology at York, states that: "Molecular biologists usually interpret genetic data by assuming a diverging hierarchy and statistically large populations. Hominin populations were small and lineages seem to have diverged and re-converged in a way that could cause molecular 'clocks' to speed up, slow down and even run backwards."
     Dr Nick Winder, of Newcastle University, explains: "In situations where the probability of producing disabled offspring was high, the 'fittest' individuals would be those that could help their offspring co-exist with this vulnerability. Those that were a little smarter, more flexible, and more compassionate would have been at an advantage."
     Isabelle says: "Like many other scientists, we believe anthropologists need an 'Extended Synthesis' able to accommodate situations where lineages re-converge, disabling genes may be flushed out of hiding, and organisms are capable of social learning that they then turn to their advantage. Our 'Vulnerable Ape' hypothesis could be part of that Extended Synthesis. Genetic vulnerability was the trigger that set our ancestors on the path to symbolic language, innovation and pro-social co-operation."
     Nick adds: "The reason every new fossil or DNA study seems to force a rethink of human evolution is that biologists are committed to a divergent, hierarchical model, with fierce competition between individual members of large populations. The new evidence tends to be much less baffling if you accept that ancient populations were often small and that early hominins had even more complicated sex-lives than our own."
     "The traditional competitive model encourages us to think of the relatively high incidence of genetic disability in our species as a threat, but the anthropological evidence suggests that the incidence of genetic disability was probably much higher in the distant past. We have good reason to believe that compassion, ingenuity and behavioural flexibility helped our ancestors cope with this vulnerability."

Edited from University of York (12 June 2015)