A Bad Day Off East Africa-Around 3 Million Yrs Ago

I do agree that humans were almost certainly responsible for the successive waves of extinctions that coincided with their first appearances in new territories. One particularly compelling bit of evidence concerns the North American mammoths. There had always been a debate over whether the primary cause of their extinction had been human over-hunting or the climate changing from glacial to interglacial conditions. I had always had my doubts about climate change, since the glaciers had retreated dozens of times before without significant elevations of the rate of extinctions, so why the huge increase on the first interglacial that included humans?

But recently the debate has been definitively resolved (in my opinion). A study has looked at the bone structure, and the population demographics, of the most recent mammoth fossils. The two scenarios under consideration would lead to very different evidence. If the culprit was climate change and resulting lack of food resources, then you would expect to see poor bone structure with evidence of numerous periods of malnutrition. You would also expect to see few infants, as the birth rate would plummet to conserve the mothers' resources, in a typical response to famine conditions. On the other hand, if over-hunting was the primary culprit, you would expect to see good bone structure, since there would be plenty of food for the few surviving mammoths, and many infants, as the population would be producing new members as quickly as possible to replace the numerous deaths from hunting.

So what to we see in the fossil record at the end of the time of mammoths? Good bone structure and many infant and young mammoths. Combined with the fact that this was also the beginning of human occupation of North America, and it's a settled case to me. Humans over-hunted the mammoths to extinction.
 
Interesting bit of trivia: A recent study of DNA in the permafrost seems to indicate that horses and mammoths survived in at least one small pocket on the mainland for several thousand years after they disappeared from the fossil record.

That probably illustrates a rather depressing point: If a species goes below a certain point, even if it survives in the short-term it may have lost enough of its diversity that it is unlikely to make it in the long-term. And even if it survives it may have lost its potential to develop in some directions.

Part of the problem is inbreeding among the survivors, which brings out genetic defects. The other part is that the surviving members of the species are too similar, which means that a disease or a ecological change that kills one of them is more likely to kill all of them.

I think of a species as being almost like a river. It's contained in an ecological niche, but constantly testing the edges of that niche, like a river testing its banks. To continue with the river analogy, the ecological niche continuously changes in small ways, just like the course of a river.

If you reduce the species enough, (a) It may no longer tests the edges of its niche and as the niche changes the remnants of the species stay at the 'deepest part of the riverbed'--the part of the ecological niche most favorable to it, and (b) competitors flood in and make it more difficult for the species to expand back into the full niche it once held.

There is a different response if the ecological niche suddenly changes at a time when the species has a healthy population. Suddenly the traits that were selected for the most are no longer all that useful. It's sort of like raising the riverbed up to the level of the surrounding countryside. The population spills out into neighboring ecological niches, searching for areas it can compete in. If it finds some it may survive as one or more new species. If it doesn't, the species dies off.

Does that all make sense? I'm probably stretching the analogy a bit, but hopefully it helps to visualize the processes involved.
 
Top