IS THERE A REASON WHY HUMAN EVOLUTION OCCURRED DURING A RARE GLOBAL ICE AGE?

13 July 2017

I have become intrigued with the fact that modern humans emerged roughly 2.5 million years ago, at the absolute temperature bottom of one of the earth's relatively rare cold climatic periods (making up only 5% of the last 3/4 billion years or so). That is, we are unarguably ice-age creatures. I've found a couple of new charts that illustrate the link between human emergence and cold climate. Interestingly, a Google search on the subject doesn't really turn up anything beyond this.

If you look at the link between human evolution and climate, you get articles explaining how climatic variability may have prompted aspects of human evolution --- tool-making, language, an enlarged brain case, etc., but we're talking in these cases about variabilities in maybe a degree or two of global mean temperature, which is small stuff if you look at the longer-term climatic record of our planet, where there is evidence of 12-25° Celsius variation in global mean temperature (today's mean temperature is in the 14-15° C range --- still near its 12° C low of 2-5 million years ago.

I also tried a search about humans as ice-age creatures, and literally all that comes up are endless articles about how hungry humans caused the extinction of the large ice-age mammals.

An examination of global mean temperatures over the past 700 million years makes clear that only about 35 million of those years were typified by today's still very low global mean temperature in the range of 12-15° C (recently warming dramatically, as everyone knows).

Note that the earth's mean temperature is much more often in the 22° C range --- with 450-500 million of the past 700 million years at or near that level. Strikingly, the transitions occur rapidly in geological time, with upward or downward spikes of maybe 8° C occurring in the space of roughly a million years (maybe less?). Positive feedback loops --- as we are seeing today --- very likely account for those geologically rapid transitions.

Keep in mind that our planet started out as a very hot fireball 4.5 billion years ago. Elevated levels of greenhouse gases kept the earth quite warm until oxygen-based life forms evolved. In turn, emerging photosynthetic life forms cooled the planet until the equatorial climate was similar to Antarctica today. When earth was a Precambrian snowball about 650 million years ago (visible on the above chart), there were no life forms to absorb atmospheric carbon, and it gradually reaccumulated, warming the planet again. It's probably the release of stored carbon in permafrost and seabeds that drives the return to 22° Celsius. Add to that today that humans are burning long-buried fossil carbon. No wonder the earth is currently warming so rapidly! 

In summary, my search for WHY humans seem to be ice age creatures has not really turned up anything at all yet. I'm not sure why the topic is not being actively discussed. One thing we do know is that humans thrive at ecosystem boundaries, so my current working hypothesis is that we also do well at climatic boundaries --- that is, trending from warm to cold and (possibly) back again. Local and regional temperature variation, which is more notable at cyclical lows than highs, would thus create the kind of ecological variation on which humans and early humans have thrived for 2-1/2 million years! 

How will humans fare when, in less than another millions years --- and possibly in only a few centuries, the planet goes back to its 22° C climatic norm (or possibly higher)? The process is gradual in individual human years, but currently extremely rapid in geological and generational terms. Our children's children will certainly be living with the ever more dramatic consequences of the current human-induced "carbon era."

As an optimist, I believe we can resolve most or all of the problems associated with a rapid return to a much hotter planet. However, we'll have to be considerably more focused than at present to accomplish the necessary ameliorations and accommodations.

Based on my reading to date, we'll do best to hold back the natural and at some point inevitable return to much warmer global mean temperatures. This will enable us to save the coral reefs as well as many other species, to preserve maximum species diversity, to keep our coastal and tropical cities where they are, and to contain northward-migrating tropical and temperate region diseases. To accomplish this, we will require fusion as well as solar power, and we'll have to put robots to work to aid us in maintaining and repairing the environments and systems we have damaged. 

This article provides the best summary of earth's long-term climatic variation that I have found: click here.
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