Neanderthals' status as modern humans' closest extinct relatives make the causes of their extinction a matter of considerable interest. A new model suggests that while the ultimate cause may remain unknown, a small reduction in fertility, maintained over a long enough time, would have been sufficient, without any sudden catastrophe.
Neanderthals were never terribly numerous, as well as being slow breeders. Dr Anna Degioanni of Aix Marseille Universite wondered if just a small reduction in births could have pushed them over the edge. She created a model for the effects of modest changes in birth rate and found an 8 percent fall in fertility would have been sufficient to bring them below 5,000 individuals in 4,000 years. Even a 2.7 percent fall would have done the same over 10,000 years. Based on her assumptions about life expectancy and age of motherhood, Degioanni concluded a small fall in fertility for Neanderthal women aged 18-20 could make all the difference, even with stable birthrates at other ages.
Some biologists consider falling below 5,000 individuals the point at which extinction is inevitable, as a species lacks the genetic diversity to avoid inbreeding. This view is controversial in the light of some apparent exceptions, but may be particularly applicable to widely dispersed species where great distances impede interbreeding.
Even smaller changes in fertility could have caused Neanderthals' demise if combined with slightly higher rates of infant and child mortality.
Degionni's model, published in PLOS ONE incorporates such factors as the migration of young Neanderthal adults between populations, which possibly sustained the last hold-outs in southern Europe after extinction had occurred elsewhere.
"This study... does not attempt to explain "why" the Neanderthals disappeared, but to identify "how" their demise may have taken place,” Degionni and co-authors said in a statement.
A decline in fertility could have been the result of reduced food supply, caused by the competition with newly arrived Homo Sapiens in territory Neanderthals had previously had to themselves. Alternatively, it is possible warfare between the two groups, or diseases unwittingly transmitted by the new arrivals, had the same effect.
Whatever the cause, Neanderthals were vulnerable because, even at their peak, they are thought to have numbered only around 70,000 across Europe and much of Asia.
Falling fertility rates today sometimes induce panicked fears of extinction for sub-populations, if not the whole of humanity. However, the Neanderthal comparison is instructive. Their entire population was less than a small city today, and yet it still probably took thousands of years for them to die out. Even if human fertility rates, which have been falling for five decades, drop below replacement levels, even relatively small ethnic populations are in no imminent danger.