How recent is our species?  Scientists estimate our species to have originated about 3 million years ago.  Our race, the human race, came from Africa about 200 thousand years ago.  We have built cities for only the last 10 thousand years or so.  Although we are a middling sized species, we have taken control of our environment.  Almost all other life on this world is in our hand.

After so brief a time, we stand on the edge.  The time of our race is brief from the geologic viewpoint:  In round figures, the earth is four and a half billion years old.  Life has swum, slithered and walked the earth for three and a half billion years.  But in a very short time we have come a long way.  And here we are, top of the heap, with the power to change everything.

Dizzying, isn’t it?  If our race disappeared, from nuclear or biological war, from a human caused change in the climate, or from some factor completely beyond our control, in only a few million years there would be no remnant of the great but ephemeral human race.

What is the distinctive feature of the humans?  Collectively, we are intelligent.  Other species have smart individuals.  The human race has dominated them all because we organized ourselves about the use of intelligence.  In order to insure our survival, we out thought every one else.  When humankind arrived on the North American continent, the great predators disappeared in our wake.  When humans arrive in the Pacific islands, the great beasts there likewise disappeared.  Most of this happened before the first cities were founded.  Everywhere we have gone, we have reshaped the world, and eliminated those species which threatened or competed with us.

The result has been a decline in the variety of life among the great forms no matter how superbly they were adapted to their environmental role:  The Saber toothed cats (a threat to us and a competitor), the mastodon (our prey), the dodo bird (easy prey), and very nearly others like the American bison.  This happened not because we had bigger claws or sharper fangs, but because we hunted in groups; we are smart together.  And you know, our brains weigh in at about 3 pounds each, an insignificant appendage.

If you came to look for humanity’s remains in a few million years, you would find nothing.  Perhaps a few scattered parts of a few skeletons of our persons, and maybe a few sections of badly decomposed highways for our artifacts.  But the exciting stuff would be the big life forms, and according to the future’s geologic record, they all would have all died out mysteriously in a few thousand years.

So... what happened?  Would our future explorer deduce our presence here?  Or would he say the absence of evidence is evidence of absence.  Poof!  Everything you see is gone.  Life on the surface of the world died out for some reason.  Hm....

The geologic record is full of such Great Dyings, as they are called.  The best known is the end of the age dinosaurs, reckoned to have happened some 65 million years ago.  A recent popular theory is the dinosaurs were killed off more or less directly when a comet smacked into the earth.  There is a good candidate crater near Mexico’s Yucatan peninsula.  The science about an impact is cogent.  Something big did hit the ground there, resulting not only in a crater in the gulf of Mexico, but also numerous smaller features.  Although the impact theory is interesting, two questions persist:  Although three quarters of life on the planet disappeared, why were certain forms spared?  Second, and more to the point, why did the Great Dying start before the impact occurred?

The evidence is the diversity of land dwelling species was steadily being reduced.  In fact, most land dinosaurs were of the grass munchers at the end of the age.  These were the agents of the creation of the grasslands.  One can imagine great herds of dinosaur migrating in search of fresh pasture to chew.  The herds of the American bison are a good point of comparison.

So with all the herbivorous dinosaurs, where were the carnivores?  There were some carnivores, to be sure.  But how came they to allow such great herds?  Perhaps there was a certain, physically un-impressive dinosaur carnivore which held the rest at bay.  Perhaps these dinosaurs had learned to work together to hunt.  In doing so, they learned to communicate and organize their intelligence.  Perhaps they built communities and ultimately they, not some judgment form the heavens, caused their own downfall.

I’m not entirely serious about this notion about the end of the dinosaurs.  There are other Great Dyings in the tale of life on earth.  The appealing thing is that this approach to the end of the dinosaurs can invoked for the other general destructions of life which happened earlier.  Only one hypothesis is necessary to explain general extinctions, rather than comets and nearby super novae and so forth.  Occam’s razor suggest we should pick the simplest explanation which fits the facts.  In this hypothesis, that explanation is the emergence of intelligence, especially intelligent, social hunting.

Intelligence gives a species an edge over all other species.  Competitors are out fought and out thought, so a physically inferior species can overcome a environmentally better adapted species.  Brain over brawn.  Looked at a bit differently, intelligence is bad for the less intelligent.  Taken one step farther, intelligence is toxic to other life forms.  Now the final step, because intelligence is toxic for every other species, and given that we emerged, in some sense, suited to our environment, our own intelligence is ultimately toxic to us.