The Case of the Missing Extinctions

Or, The Pleistocene Extinctions Pattern Implications For Understanding Human Evolutionary Nutrition

Prior to fifty thousand years ago, before the Pleistocene extinctions, ancestors of modern humans lived in Africa.   Modern man, Homo sapiens sapiens, emerged in Africa, before these extinctions. 

As explained by geneticist Spencer Wells in the documentary film Journey of Man, by tracking the Y chromosome, we now know that all modern humans are descendants of a few men among the ancestors of the !Kung people currently living in the Khalahari desert. 

But in ancestral times, the Khalahari was not a desert. During the Pleistocene ice ages, “the great deserts of North Africa and Western North America today were mostly vast grasslands with large permanent lakes and abundant game animals.” [1]  

Many believe that humans evolved primarily by hunting those land animals, and take the Pleistocene extinctions as evidence that humans evolved as top predators of grassland animals in Africa.

In "Of mice, mastodons and men: human-mediated extinctions on four continents" [2], Lyons et al provide several convincing lines of evidence and reasoning that indicate that humans must have caused the Pleistocene extinctions.  To simplify, no other known natural phenomenon (such as climate change) could account for the sudden selective extinctions of the megafauna, without simultaneous extinctions of smaller species. 

But there is something very interesting about the pattern of the Pleistocene extinctions.

The following graph from Lyons et al [2] shows the pattern.  The hatched bars show the species exterminated in the late Pleistocene.  As you move from left to right, the bars represent increasingly larger species.   Notice anything unique about Africa?

On every continent where humans were an invasive species, the largest animals went extinct.  

Source:  Networkroundtable.org

But not in Africa.

There, most of the large body species survived.   

Source:  Vivagoal.com

It appears that as humans invaded Australia and the Americas they succeeded in exterminating the fat megafauna, but humans who remained in Africa did not, and many, including giraffes, elephants, and rhinos,  have remained to this day.  Why?

Lyons et al have a suggestion:

“The lack of extinctions in Africa (Fig. 1) is especially notable given the long history of humans on this continent. The co-evolution of man with the African megafauna may have resulted in the evolution of effective anti-predator behaviours (Diamond, 1984; Martin, 1984).”[Italics added]

In other words, upper Paleolithic humans did not exterminate the megafauna of Africa because the African megafauna had plenty of experience with and knew how to avoid human predation.  Just the smell of humans probably sent them running.

Source: bioweb.uwlax.edu

  

In contrast, animals in Europe, Asia, Australia, and the Americas most likely evolved independent of human predation.  This meant that they were, for humans, relative to African fauna, ‘sitting ducks.’ 

Thus, considering the co-evolution of humans with African fauna, the fact that the extinctions did not occur in Africa supports the hypothesis that humans, as an invasive species, caused those extinctions on other continents.

It also suggests that the invasion of other continents allowed humans to pursue a subsistence strategy––hunting large, fat anmals––that would not have been as successful in Africa.

Moreover, we could reasonably take the statement by Lyons et al and modify it slightly:

The co-evolution of man with African fauna (mega or not) most likely resulted in those fauna evolving behaviors effective for avoiding human predation.

This further suggests that the human exodus out of Africa into other continents may have resulted in a significant change in human diet composition, after H. sapiens sapiens had already emerged.  In other words, it suggests that the later Paleolithic diet of humans outside of Africa may have contained much larger amounts of land animal meat and fat than would have been possible in the early Paleolithic among the far more wary wildlife of the homeland.

Let me put it this way.   An animal adapted to a diet obtained in the Africa invades a new ecosystem.  In this new ecosystem, this invasive species finds that hunting is a lot easier than it was in the homeland, because the animals are relatively oblivious to the danger presented by humans.  Consequently, in accord with optimal foraging theory, this animal goes for the easiest calories possible:  large animals that don’t know that they should run away from anything that smells, sounds, or looks like a human.

If this new diet supports health adequately to allow reproduction for a majority of individuals, but disrupts homeostasis just enough that it gradually induces metabolic disorders that emerge over time, past the time of reproduction,  or shortens the average individual’s lifespan by inducing chronic diseases, the new diet will not select against those not adapted to the new diet, nor produce a population better adaptated to the diet.

I might even call it the upper Paleolithic dietary revolution. 

Now, let’s consider Africa again.  As noted above, the great deserts of Africa were grasslands with large “permanent” lakes (well, permanent during those ages).  If hunting land animals was not as easy in Africa as in Eurasia and the Americas, what about hunting animals living in those lakes?  Would they all catch the smell of humans or run away easily?

Lacustrine, riverine, and wetland environments are very rich in food resources, both plant and animal, including animals that don't move very fast (shellfish).  Foraging in such environments might just provide greater return on investment than possible on a grassland.

Some people studying human evolutionary nutrition believe that the archaeological, cultural, nutritional, biochemical, and medical evidence points in the direction of humans evolving in those econiches, where shellfish, fish, and amphibious animals could have been the primary sources of animal foods.  These animals have nutritional properties significantly different from  those of savannah animals—the protein, fats, and mineral contents of water animals all differ significantly from land-based animals, in very interesting ways relative to specifically human nutritional requirements, particularly for the nervous and cardiovascular systems.

In fact, the earliest human fossils are consistently found associated with lacustrine or marine fossils indicating humans inhabiting niches incorporating a land-water interface:  wet woodlands, flood plains, wetlands, rivers, lakes, and coastlines.  In these two segments of the Journey of Man, we learn about archaeological evidence of humans living along the African coastline and consuming large amounts of seafoods, leaving large heaps of seashells as evidence (segment starts about 5:00 of first video):

 Also, and again, as explained in the  Journey of Man, the first exodus of humans from Africa appears to have followed a path along the marine coastline to Australia; it does not track through a grassland.   And, in this segment, we learn that some of the first humans in Australia lived in a lacustrine environment (now a desert), described as "quite a rich environment," eating fish (leaving hearths and fish bones as evidence):

Indeed, in the two segments below, Wells point out that, at the end of this journey, to get from Indonesia to Australia, it appears that people had to find a way across 150 miles of open ocean (segment starts at about 3:20 of the first video).  

The animal capable of this seafaring--150 miles of open ocean with only the most primitive of boats--would have had to have been very familiar with and physically very well adapted to a marine environment.  Not a likely accomplishment for a mammal specialized in exploiting the dry savannah environment. 

Which leads me again back to Michael Crawford and David Marsh, co-authors of Nutrition and Evolution.   Crawford, a biochemist, has devoted his life work to study of the effects of fats on human physiology, and has a bit to say on the subject and its relation to human evolution.  Some others have something to say about the differences between land and animal foods with regard to human brain nutrition.

But for now I have run out of time.  To be continued.