An evolutionary sidenote

This post has nothing particular to do with climate change, but I thought it might be worth pointing out a recent study analyzing the evolutionary history of Africans and African Americans.

Africa is the source of all modern humans, originating there a few hundred thousand years ago and spreading across the globe within the last 100,000 years. The authors analyze DNA from 113 populations of Africans from across the continent and find that they descend from 14 ancestral groups (with the highest within-population diversity worldwide) and find significant associations between genetic and geographic (as well as linguistic) distance in all regions of Africa. These groups later interact with each other to create the distinct populations that exist today. The study of African genetic diversity will be important for reconstructing African and African American population histories, as well as the genetic basis of diseases prevalent in Africa.

As another sidenote, there is a new blog called Darwinaia which will have focus on paleontology, history of science (particularly evolutionary related stuff), so if you’re into that, check it out.

3 responses to “An evolutionary sidenote

  1. sweet article, and thanks for the nod on here!

  2. In some ways, I applaud the sense of urgency that accompanies the perceived need to do something to affect climate change. The need is there in more ways than you presently know. But the means is another matter entirely. The Akkadian Empire under Sargon (2,300-2,200 BC), mankind’s first empire ever, succumbed to climate change that happened rather suddenly. A 300 year long period of drought struck this nascent civilization and toppled what turned out to be only a 100 year empire. The Old Kingdom of Egypt and the Harappans of the Indus Valley suffered a similar fate 4,200 years ago, succumbing to an abrupt drought that ended those civilizations, with Egyptians “forced to commit unheard of atrocities such as eating their own childrenand violating the sacred sanctity of their own dead (Fekri Hassan, 2001)”. The Mayans had pretty much the same luck with three periods of extreme drought at 810, 860 and 910 AD. Sadly just two years after the drought which saw 95% of the Mayan population gone, wet years returned to the Yucatan. A reconstruction from fossil algae in sediments from Drought Lake in North Dakota of the past 2000 years found that dry conditions were far and away the rule in the High Plains, with the Dust Bowl conditions of the 1930’s one of the lesser dry spikes found in the record. Half of the warming that brought us out of the last ice age (the Wisconsin) occurred in less than a decade.

    There were 23 Dansgaard-Oeschger events between this interglacial, the Holocene, the interglacial in which all of human civilization has occurred, and the last one, the Eemian, in which the first fossils of Homo sapiens are to be found. D-O events average 1,500 years, and have the same characteristic sawtooth temperature shape that the major ice-age/interglacials do, a sudden, dramatic, reliable, and seemingly unavoidable rise of between 8-10C (on average), with outliers up to 16C (taking only from a few years to decades) then a shaky period of warmth (less than interglacial warmth), followed by a steep descent back into ice age conditions. We know with absolute certainty that these events happen, with evidence of D-O events extending back some 680 million years. We do not know yet precisely what causes them. What we do know is that the past 6 interglacials (dating back to the Mid Pleistocene Transition) have lasted roughly half of a precessional cycle, or 11,500 years, which just happens to be the current age of the Holocene. What we know is that N65 latitude insolation values are very close now to what they were at the close of the Eemian. What we also know is that GHGs seem to have played only a spectator role to all of these natural transitions, with temperature changes leading GHG concentrations by a considerable margin of time. What we do not know is if anthropogenic sourced GHGs can trigger a climate change event. What we do know is that earth’s climate is bimodal, cold (90%) and warm (10%), with the transition times (such as at the end of an interglacial) well known from proxy records to be quite sensitive to forcings we do not yet understand, and the forcings we have identified seemingly incapable of producing the responses we see in the paleoclimate record. Including the recent paleoclimate record.

    The Holocene is studded with the Younger Dryas, a 1,300 year near instantaneous return to ice age conditions some think resulted from a change in meltwater flow from the Gulf to the North Atlantic, shutting down the Gulf Stream circulation. Almost as suddenly we came out of it. Between 6,000 and 7,000 years ago, a period known to geologists and paleoclimatologists as the Holocene Climate Optimum sea levels peaked about 6 meters (about 20 feet) higher than today, and during the Eemian Optimum, some 20 meters (about 60 feet) higher than today. During the seven post MPT ice ages, sea levels dropped some 100 meters below present, the water tied up in the miles thick ice sheets that have spread in North America as far south as Kansas. These are just some of the facts of the abrupt climate changes which we, as Homo sapiens, have experienced. Global Circulation Models, of which the IPCC references 23, have yet to reproduce a single known abrupt paleoclimate change. The models produce predictions based on a variety of input data and complex equations which few of us would understand. But for all the complexity and investment, they are just predictions.

    Belief in, and acting as a result of, such predictions has opened up what may be the first chapter in faith-based science. Understanding the history of climate change provides a factual understanding of far more alarming climate changes that have actually happened, with sea level changes and temperature shifts that dramatically overshadow any faith-based prediction you have yet heard.

    What might be quite ironic is that if GHG predicted global warming is in fact real, and, at half of a precessional cycle, we are near to the cliff of the next natural shift to an ice age, we may find ourselves needing to generate as much GHGs as possible to ease our transition into the next ice age. So as I said at the beginning, doing something about climate change is by no means a bad thing. Doing the right thing might actually be quite another. The ice ages and associated interglacials are well known to be paced by the eccentricity, obliquity and precession cycles in earth’s rickety orbit. These we will do nothing about. D-O events show strong evidence of being tied to the 1,500 year cycle of solar output, something we cannot change.

    So be ever thoughtful of both facts and predictions before leaping to a conclusion. It was in fact a LEAP that terminated the last interglacial, the Late Eemian Aridity Pulse which lasted 468 years and ended with a precipitous drop into the Wisconsin ice age. And yes, we were indeed there. We had been on the stage, as our stone-age selves, about the same length of time during that interglacial that our civilizations have been during this one.

  3. OMG loved reading this post. I submitted your feed to my google reader.

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