No Intelligent Life Here, Captain - Part 1

You know the plot. People of the future land on a new planet, check it out for intelligent life, and conclude there is none—but miss something (someone?) sinister hiding in the bushes and watching them. Later, the local intelligent life attacks.

Another version doesn't involve interplanetary travel but perhaps exploration of a new continent, or a remote island never-before-seen by humans. The result is always some sort of competition, inter-species conflict, and mayhem.

What is perhaps surprising is that this scenario has actually happened, repeatedly, in our history. Though there is now only one sort of two-footed, unfurry intelligent life on earth, this was not always so. Many times in the history of our evolutionary lineage, there have been at least two and maybe more species of hominid alive at the same place at the same time. And only one species has survived: modern humans, Homo sapiens, us.

The last such showdown occurred when early modern humans met Neandertals in Europe. Accusations have been made of warfare, cannibalism, brutal genocide, but these are very poorly supported by hard evidence. We don't know if interpersonal violence was the norm, although the bones of each type of hominid reveal healed fractures and injuries, because life was perilous in those days in any case. The pattern and frequency of injuries on Neandertals parallel those of today's rodeo riders, who typically accumulate a painful array of injuries while handling large and unwilling animals. Probably catching and killing large unwilling animals accounts for most of the injuries seen in Neandertals.

But you don't have to kill or actively exterminate another species to win in evolutionary terms. All you have to do is outcompete the rival and your species can survive while another goes extinct.

What form does this competition take? One good ploy is breeding faster, so your population becomes overwhelmingly larger and therefore ecologically dominant. Another is exploiting the same resources—food, water, shelter, and the raw materials for making tools spring to mind—and being better at it than the other species. We have no direct proof that Neandertals and modern humans ever met face to face, but we know they competed with each other. And we know who won.

Here's the scenario. Archaic humans evolved in Africa about 1.7 million years ago and some populations expanded their territory out of Africa across Europe and Asia. Others stayed behind in Africa. (Of course, neither had any clue they were moving between continents. They were probably just following game and other food sources.) By about 230,000 years ago, the Eurasian hominids had evolved the familiar Neandertal features: stocky build, powerful limbs, large brows and an elongated head. They successfully hunted large game animals—wild horses, bison, wooly rhino, several sorts of deer, and aurochs, a form of extinct cattle—with an array of stone tools. By about 200,000 years ago, the African hominids had evolved into the earliest modern humans with anatomical features like our own. They too were successful hunters of large animals that used stone tools. The showdown came when modern humans began to expand out of Africa into Eurasia, the vast territory already inhabited by Neandertals. The time was about 50,000 - 45,000 years ago.

What happened? Why did our lineage prevail and Neandertals go extinct in a habitat where they had already lived successfully for roughly 200,000 years? In geological terms, the extinction was rapid: it took modern humans only 10,000 years or so to force their evolutionary "cousins" into extinction over the entire vast range of Eurasia

A staggering new glimpse of how this occurred comes from a brilliant new study by Paul Mellars and Jennifer French, of Cambridge University, published recently in the journal Science.

They carried out a meta-analysis of Neandertal and modern human finds in the Dordogne region of southwestern France, which is known for the density of its archaeological sites. The area is large, approximately 75,000 sq. km. The time range sampled is from 55,000 - 35,000 years ago, which spans the entire Neandertal-to-modern transition. They collected information on 164 separate sites.

What is particularly clever about Mellars' and French's study is that they looked at three independent measures of the success of each hominid, divided up into 1,000-year intervals so that the numerical dominance of the modern human sites would not warp the results.

The simplest analysis shows that the number of sites almost doubled through this transition. Of the 164 sites from this period that are known in the region, 56 (34%) are Neandertal (identified by Neandertal fossils or, more often, stone tools associated with Neandertals) and 108 (66%) are modern human. With the very same set of resources for supporting their populations, modern humans left almost twice as many sites. Corrected for the time duration of those sites, modern humans left about 2.5 times as many sites as Neandertals.

Not only did modern humans leave a lot of sites, but discoveries at those sites showed the modern human sites were more intensely occupied. For one thing, more people need more tools. Early Neandertal sites had an average density of 6.6 stone tools in every square meter over a thousand-year span. In later Neandertal sites, the density had increased almost 50%, to 9.7 tools per square meter per thousand-years. But modern humans sites had a much greater density of 17.6 tools per square meter per 1000 years.

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Another indicator of the comparative success of each group lay in the food remains—fossilized animal bones—that accumulated at these sites. Mellars and French calculated how many individual animals were represented by the bones and then transformed this number into the weight of meat on those animals. Here, too, the difference was impressive. The meat weight densities per 1000 years from modern human sites were about 1.8 times greater than those from Neandertal sites. Both groups were hunting the same range of animals in the same physical environment, so the implication is that modern humans had more hunters or better hunters, or both.

The last proxy for population size used by the team was the size of the sites, since more people will create larger sites. This is a tricky measure, since these sites have been excavated by different researchers at different times and archaeologists do not always excavate the entire site, preferring to leave areas for future researchers with better techniques. Any measure of the size of an archaeological site has to be treated as a minimum estimate.

Nonetheless, the team found the same pattern. Though both groups left some small sites, where the difference was revealed was in their largest sites. The largest early Neandertal sites were 100 square meters and the later ones were 250 square meters. But the four largest sites created by modern humans were between 500 and 600 square meters: between two and three times larger than the largest Neandertal sites.

What does all this mean? Because these different indicators of population are independent of one another, the results should be added together and not averaged. By the time Neandertals were on the verge of extinction, roughly nine times as many modern humans were occupying the area where Neandertals had once lived. Compared with Neandertals, modern humans left 2.5 times as many sites, created sites that were 2-3 times larger, made 2.5 times as many stone tools, and took almost twice as much game from the same environment.

This is a staggering dominance of an invasive species over an invaded one. Invaders often have a substantial advantage. Obviously modern humans were substantially better at getting and using the resources of the Eurasian environment than Neandertals. More tools meant more game; more game meant more meat; more meat meant better nutrition for humans, which almost certainly caused the birth of more babies and more babies that survived to adulthood.

Is this unusual? Not really. About 40% of all extinctions where a cause is known involve invasive species forcing the resident species into extinction. I can hear the cry "but why?" ringing out in my mind, and I hope yours. Why did we survive and Neandertals go extinct?

Come back for part 2 and some possible answers. You may be surprised at what might be the basis for the survival of our kind.