A Short History of Evolution: Natural Selection (Part 2)

Comparison of Modern Human and Neanderthal skulls from the Cleveland Museum of Natural History. Photo by Matt Celesky via Flickr. Derivative work by KaterBegemot.

This article is part of Carl Coon’s ongoing “A Short History of Evolution” series. Click here to read all entries in this series.. 

Blips on the Screen

We know a lot about what exists today and we can see back, with some help from the earth sciences, quite a way into the ancestry of current flora and fauna. But there is an incredibly long period before that about which we know very little. Here’s a quick reference to some of the scraps of knowledge that we do have.

There is geological evidence that the Earth had little or no oxygen for much of its existence. Oxygen levels then spiked about two billion years ago, and fluctuated after that. There are possible correlations here with the evolution of early life, but it’s too early to say much more; it is a new field of inquiry, one that is just getting started.

Then there is the Burgess Shale and the issue of punctuated evolution. The Burgess Shale is a rock formation in Canada that contains a bewildering variety of fossils created about 500 million years ago. This extraordinary variety led Harvard professor Stephen Jay Gould to formulate a theory of punctuated equilibrium1, which held that evolution proceeded by sharply defined steps, rather than in a smoother, more linear gradient.

If there is any merit in the present survey, it should put that argument to rest. Evolutionary progress is always linear, but it can come in varying tempos, and when seen at a considerable distance in time, a linear progression can look much like a step. The issue Gould raised is essentially moot, because it comes down to the semantic issue of when a step is not a step. We shall find out as we continue this narrative that as we approach what appears to be a sharply delineated boundary, we see not a line but a zone, within which various intermediate processes are happening that make possible what we earlier saw as an abrupt change. It all depends on the timescale you are using.

Another major event was the Permian holocaust. About 65 million years ago, a giant meteor crashed into Yucatan and created something like that nuclear winter that doomsday types were predicting during the height of the Cold War.2 The result: massive species extinctions, especially the dinosaurs. The whole top of the food chain was lopped off, and then some. Some of the smaller saurians became birds, in an evolution that had already begun, while mammals took over on the ground. The mammals were little fellows, by comparison with their gigantic predecessors, but with a different design structure, better equipped to survive in new and frequently harsher environments.

That brings us up to date, in a sense. We’ll fast forward to the era of Homo sapiens once we’ve taken a quick look at Neanderthals and some of our other close relatives, now extinct. We’ll find the path to language and culture that opened the way to civilization as we know it.


Ancestral Cousins

A great deal of work has been done in recent decades to establish human origins, but important issues remain unresolved, and the chart is still full of blanks remaining to be filled in. Here is a very brief (and necessarily controversial) summary of the famous

“missing links” between our own species and its immediate predecessors.3 We are still dealing with a process that is almost entirely biological, with natural selection dominating, and cultural selection still waiting for its curtain call.

Several million years ago a line ancestral to the modern chimpanzee split into three main components, chimp and bonobo on one side, and the genus Homo on the other. Homo’s further evolution was linked to major changes in the climate and topography of eastern and southern Africa, when much of those regions changed from tropical forest to savannah. The chimps and bonobos stayed in the forests and evolved very slowly, since they were already well adapted to an environment that was also changing very slowly. But Homo was in an environment where, in order to survive, massive changes were required in both behavior and physical structure. The most obvious physical change, and the one most directly related to the changed environment, was to become primarily bipedal rather than arboreal. This transition was followed (or accompanied) by a cascade of other changes. Many of these changes involved bone structure that paleo-anthropologists have studied in detail from skeletal evidence. Some other changes, like those involving use of stone implements, fire, and dietary habits, can also be inferred from the archeological evidence.

During this period a variety of experimental models emerged. They all walked upright, and experimented in somewhat different ways with necessary collateral changes, mainly in the head, pelvis, and feet. Some of these pilot projects were more robust than others. Bipedalism freed the hands and made it easier to manipulate objects. There was a trend toward bigger brains. We also know from the archeological evidence that at least some of the variants had fire and crude stone implements. Since they were all competing for much the same ecological niche, it became like a race, or lottery: one winner, many losers.

At least one of these variants evolved along lines that gave it an unusual capacity to adapt to new environments. About two million years ago, it spread out of Africa into far reaches of the Eurasian continent. Over many generations its descendants evolved into Neanderthals (in Europe) and similar subspecies farther east (whom we refer to, collectively, as Homo erectus). These populations, sometimes called primitive humans, were organized in kin-based groups, and there is evidence that they were acquiring the rudiments of what we now know as culture. But they remained essentially conservative. Their armory of stone tools didn’t change much over a very long period, whereas the sapient humans that succeeded them took less time to develop more sophisticated flint knives and a wide variety of other implements.

So what do we see when we survey what we know about the Neanderthals and their cousins? I suggest that we see a species that had evolved to the upper limits of its physical potential. They had big brains and were capable of living in cold as well as warm climates, but they were conservative, in that they adapted slowly, by comparison with the sapient humans that followed them. They were pressing on a ceiling, but had not yet figured out a way to break through it.

About 50,000 years ago a group of people that looked like us and had evolved over the previous hundred thousand years burst out of Africa and quickly overran a great deal of Eurasia, including Southeast Asia and Australia as well as Europe. The earlier populations, as far as we can tell, disappeared. Homo sapiens won the race.

[1] Wonderful Life, the Burgess Shale and the Nature of History, by Stephen Jay Gould, W.W.Norton, 1989

[2] Wikipedia has several good summaries of this event. See, for example, here.

[3] Stringer, Christopher, Lone Survivors, 2012. The serious literature devoted to exploring our ancestral roots as a species is extensive, and new studies appear almost weekly. I find Stringer’s Lone Survivors to be the best overall source in that it is comprehensive, authoritative, and current–at least for the moment. It covers both the experimental bipedal models and the transition from Neanderthal to sapiens.

This article is part of Carl Coon’s ongoing “A Short History of Evolution” series.
Click here to read all entries in this series.