We had a bone to pick

Here's the BBC report of a story relatively out of our realm of expertise (that is, we're totally ignorant in this area).  An archeological find that was discovered 30 years ago, and just reported in Science, has been dated, and casts some doubt on a struggling theory of the age of human habitation of the Americas, and the apparent subsequent disappearance of many mammals, like mastodons.

Recent studies have strengthened the case that the makers of Clovis projectile points were not the first people to occupy the Americas. If hunting by humans was responsible for the megafauna extinction at the end of the Pleistocene, hunting pressures must have begun millennia before Clovis.

Mastodon rib with the embedded bone projectile point. (A) Closeup view. (B) Reconstruction showing the bone point with the broken tip. The thin layer represents the exterior of the rib. (C) CT x-ray showing the long shaft of the point from the exterior to the interior of the rib. (D) The entire rib fragment with the embedded bone projectile point. (From Science, subscription required.)

It was once thought that 'we' (that is, Native Americans) got here around 12,000 years go.  Shortly thereafter, many large animal species disappeared.  People using a particular projectile style called Clovis points were argued by some to have hunted mastodons and other large species to extinction.  Others argued that we couldn't have driven such species to extinction, and it must have been some other kind of ecological changes that were responsible.

Accumulating evidence is that humans got here somewhat earlier than had been thought, but it was still questionable what happened to the Big Game species.  The latest evidence dates a mastodon killed (or at least wounded) by a human-made projectile to an early time and suggests that humans did, at least quickly learn to hunt these beasts, before the appearance of Clovis-style tools.

So, we arrived and bye-bye mastodons!  Now, humans had been in the Old World for millennia upon millennia, and it's no surprise that those who could manage in the arctic climate, work their way across the Bering land bridge (presumably in part by hunting migrating game) knew how to hunt the ground-dwelling species they found here.  They had to have eaten something, after all, even in the winter!

The interesting thing to us, who are not archeologists and haven't been involved in debates about timing, Clovis, and so on, is whether this find just shows that we hunted big things like mastodons, or whether we were responsible for their total demise.  Or whether climate or other changes (related to those that produced the Bering land bridge?) were in the main responsible.

Let's assume humans are guilty.  How can this be?  How can the relatively sparsely distributed hunting bands at that time (and there's no evidence for anything more dense and sophisticated than such groups) have tracked down and killed off all of several species?  After all, the ancestors and contemporaries of the New World settlers didn't exterminate all large land mammals in the Old World.  And many species in the New World (bison, deer, and others) were doing very well for long time after our first appearance.  Bison disappeared, current wisdom has it, only when the Indians gained horses from atop which to slaughter on a large scale.

Could this be the phenomenon of new immigrants doing in species that had not adapted to them as predators? If so, what was different about that, since clearly there were also predators in the New World, so it wasn't Shangri La for mastodons.  Or were mastodons too, well, mammoth for sabre tooth tigers to mess with?  This new find shows earlier evidence of human predation but does it change these questions themselves?   Presumably the professionals (not us) will have things to say about this.