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December 10th, 2009
Expert Blogger: A Spark or an Ember? by John Shea

For the Human Spark crew, one of the coolest things about traveling all around to talk to scientists is the chance we get to cross-pollinate ideas between researchers in widely disparate fields. We frequently discover interesting but unexpected points of overlap. And sometimes a visit from our film crew can jostle a scientist’s thinking about his own work. In the case of John Shea, the way we posed our questions about the human spark got him pondering the evolution of our human uniqueness in a new way. Here he shares his thoughts.

A Spark or an Ember?

By John J. Shea, Anthropology Department, Stony Brook University

John Shea with some of his stone age technology. Photo: Larry Engel

John Shea with some of his stone age technology. Photo: Larry Engel

Filming The Human Spark with Alan Alda led me to question some of the assumptions we make about the evolution of human uniqueness – the metaphorical “spark” in the title of this series.  Most anthropologists assume that the qualities that made humans unique evolved recently and only among members of our species, Homo sapiens.  But what if this assumption is an accident of history?  Might the things we think make us unique actually be characteristics we share with other hominins who are now extinct?  A spark can be the beginning of a fire, but it can also be the last ember of a conflagration.  What if our spark is not the start of something new, but rather the culmination of a long-running evolutionary trend?

In evolution, only differences matter.  The differences between humans and our nearest primate relatives, the chimpanzees, are not subtle.  We differ in locomotion, in how we use tools, in our diets, in how we get along with one another.  In virtually every way anthropologists care to make comparisons, we differ more from chimpanzees than chimpanzees differ from other apes.  Genetic studies suggest these differences accumulated over nearly 6 million years.  If all one had to work with were comparisons of the morphology, genetics and behavior of living species, one could not help but conclude that we are special, that we humans have a “spark” that chimpanzees and other apes do not.

But we know there is a fossil record for human evolution, and it tells a very different story.  Humans evolved in the Pleistocene Epoch (2.5 Million to 12,500 years ago).  This was a momentous period in the evolution of life on Earth.  It was a great time to be a hominin.  The term hominin refers to the group of bipedal primates that includes humans.  Two million years ago there were at least three major groups of hominins, Australopithecus, Paranthropus, and Homo, all living in Africa.  Each of these groups comprised at least two and almost certainly more distinct species.  For much of the Pleistocene, there was more than one human-like species walking the Earth at any one point in time.  As recently as 40,000 years ago, there were at least three, Neandertals in Europe, Homo sapiens in Africa and Asia, and Homo floresiensis in Indonesia.  Today there is only one hominin species, us.

Being the sole remaining contestant of “Survivor: Pleistocene” influences our ideas about our “human spark” and about the nature of human uniqueness.  Our “human spark” looks special to us because we cannot compare it directly to those of our extinct hominin relatives.  The evolutionary gulf between living apes and us is a recent evolutionary condition.  If one takes extinct hominins into account, the gulf between humans and apes will appear not so wide, because it would be populated by countless ape and hominin species.  Each species would have had its own “spark,” its own uniquely evolved characteristics, and those characteristics would differ with evolutionary distance.  Species with a recent common ancestor would be more similar to one another.  Our “human spark” would be very similar to that of the Neandertals, less similar to that of Homo floresiensis, and very different from that of Australopithecus.  Our “human spark” would still differ from those of living apes, but along a complex continuum of ape and hominin variation.

What caused the reduction of species in the Genus Homo?  The simple answer is that we do not know, but we can venture some well-founded hypotheses.  Climate change and habitat loss almost certainly played a role, as they do in recent animal extinctions.  Many early hominin fossils (particularly australopithecus and paranthropus) are found in woodland habitats.  Such woodlands have been losing ground to grasslands for the last two million years.  Predators may have played a role.  The carnivores that preyed on australopithecines and paranthropines were mostly solitary felids (large cats like leopards).  The Pleistocene witnessed the evolution of large social carnivores, like lions and wolves.  These carnivores may have caused problems for some hominin species, either preying on them directly or out-competing them for access to meat and fat from large animal carcasses.  Bad luck may have played a role, as well.  Neandertals lived in some of the coldest habitats ever occupied by primates during a period of rapid, near chaotic climate change.  Their extinction, though tragic, is not particularly surprising.

Paleoanthropologists have been strangely reluctant to consider the role of competition among hominin species in the evolution of the Genus Homo.  Yet, competition is the engine that drives evolution.  In evolutionary competition, your most formidable rivals are those to whom you are most closely related.  Homo sapiens’ evolutionary success must have come at the expense of other hominin species, most likely those closely related to us.  One can see proof of this in a pattern that occurs in the fossil record.  In region after region, the first appearance date of Homo sapiens fossils is closely correlated with the last appearance dates of other hominin species.  There appear to have been some places where other species “held their ground:” Neandertals in southern Spain, Homo floresiensis in the forests of Indonesia, but these are exceptions, and in neither case is there clear and convincing evidence for long-term, face-to-face encounters between our species and other hominins.

This is why I am skeptical about arguments that early Homo sapiens killed off the Neandertals and other hominins.  It is not that I think they were necessarily good-natured.  Their moral sentiments probably varied widely, just as ours do.  Rather, I think they just did not encounter other hominin species often enough for the benefits of sustained conflict to exceed the risks and costs.

So, why did Homo sapiens survive and other hominins become extinct?  One key to our “human spark” is our uniquely broad ecological niche.  An ecological niche is the network of predator-prey relationships between one species and other species.  In evolutionary competition, generalists (species with a complex niche) always beat specialists (species with a simple one).  Homo sapiens is the ultimate generalist.  We sustain ourselves on animal prey ranging from snails to elephants, on birds, fish, and countless plant foods.  Much of this niche breadth reflects recent innovations, such as agriculture and pastoralism.  I am increasingly convinced that there was an earlier “revolution” in our ancestral human niche, one underwritten by the use of projectile weaponry.  Projectile weapons, such as the bow and arrow are niche-broadening tools.  The same bow that can launch an arrow at a fish or rodent can bring down an elephant, when it is tipped with poison.  Projectile weaponry is uniquely human and culturally universal.   We are the only species that uses projectile weaponry, and no human society has ever abandoned its use.

In seeking the origins of human uniqueness, I think it is absolutely crucial for archaeologists to work out when and where humans began using projectile weaponry to broaden their ecological niche.  Right now, evidence in the form of stone points similar to recent arrowheads is strongest in equatorial Africa, the region in which Homo sapiens first evolved.  The strongest such evidence dates to between 50,000 to 100,000 years ago, but as noted in the Human Spark, new discoveries will almost certainly push these dates back further.  I do not think that projectile technology alone explains human uniqueness.  Nothing in evolution is that simple.  Yet, projectile weaponry is an interesting piece of our human evolutionary puzzle that has not received the scientific attention it deserves.

Some academics look down at television programming as “merely” entertainment.  I disagree.  If you take public money for your education (as I did), and expend such funds in your research (as I do), you have a moral obligation share the fruits of your studies as broadly and effectively as possible.  Far more people will view the Human Spark than will ever listen to my academic lectures or read any of my scientific papers.  The most effective way to show how scientific research about human evolution matters is by working with people like Alan and his colleagues to create a thought-provoking program like the Human Spark.

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