Wednesday, December 14, 2016

No bridge yet discovered

kw: book reviews, nonfiction, science, evolution, human origins

One popular conception of evolution is the giant leap: A rabbit wakes up one morning to find that it is a deer; a monkey goes to bed a monkey and wakes up an ape, or maybe a human. This was a popular view in the early Twentieth Century, and the occasional story appeared in which someone used radiation to "evolve" into a higher being with massive intelligence and penetrating wisdom. It lingers in genres such as the Fantastic Four or X-Men, and even Spider Man, in which some mysterious energy or chemical influence suddenly "evolves" ordinary folks into super beings. I don't pay as much attention to these stories as most folks, but it seems to me that the only "superhero" story that doesn't depend primarily on some kind of magical transformation of this type is Batman.

We do like our catastrophes, and we do like the solutions to disastrous problems to be simple, complete, and in particular, FAST. Need I belabor our addiction to the 30- and 60-minute TV episode formats, or two-hour (more or less) films? Emotionally, we need fast solutions, because if we don't solve the problem of "one death per person, no exceptions", in a matter of a few decades, we are all to be thrust into eternity, not even knowing if there is an eternity. Of course, as a Christian, I have a spiritual answer for that. But as a Humanist, I know that most folks don't really believe in an afterlife; instead they hope against hope that there really is one, and that it isn't Hell.

Humans are the longest-lived of the mammals, although perhaps some species of baleen whale might live longer. A few slower-living critters are known: A turtle that was collected while Darwin was living died last year; some species of clam were recently shown to have individuals aged as much as 500 years. But among animals that can recognize themselves in a mirror, only humans routinely live more than 60 years. But compared to evolutionary time or geologic time, this is very short and inevitably forces our mind into rather short-term thinking.

Can any of us, without great effort and practice, comprehend time spans much greater than 50 years? Aficionados of genealogy gradually come to comprehend spans of a few hundred years, perhaps a thousand or so: a few Europeans and Euro-Americans can trace their family tree to Charlemagne, some 1,200 years ago. A few cities (Damascus comes to mind) have been occupied more-or-less continuously for 3,000 years or more. How can any of us comprehend a million years or more? The "young earth" Evangelicals, who allow no more than 6,000 to 10,000 years for the entire age of the Earth, are onto something. That is the limit most people's minds can reach, and a span of 10,000 years is practically meaningless; how much more unfathomable a million or a billion?

The basic unit of the evolution of vertebrate species is one million years. That seems to be on the shorter side of the survival of a species of mammal, bird, or reptile. The longer side is three to five million years (I may discuss marine invertebrate species, some of which appear to survive for tens to hundreds of millions of years, on some another occasion). Interestingly, the survival of primate species seems to be inversely correlated with body size. Species of monkeys and small apes weighing less than around ten kilograms may go on for five to ten million years. Larger apes seem to speciate more frequently and vanish sooner. The human ancestral apes and proto-human species of genera such as Homo, Australopithecus, Ardipithecus and so forth seem to stay around between 100,000 and 500,000 years. Of course, the fierce competition among paleoantrhopologists to be remembered for naming new species and genera has resulted in some over-splitting, so future scientists will most likely lump together many putative species .

I also wonder: If there were equally huge prestige associated with naming each new species of fly or snail or nematode, would the number of named species multiply, and the duration of fossil species accordingly shorten? A side note: I spent the last year taking inventory of a few thousand specimen lots of river-and-pond snails of the family Pleuroceridae, at the Delaware Museum of Natural History. The collection includes about 300 names species in a dozen genera. This is a winnowing (i.e., lumping together) from about 1,000 named species described prior to 1980 or so, and more recent work using DNA methods is finding that many "species" are regional variations of more widespread species. A specialist in the field told me recently that there may actually be no more than five or six actual species in this family! To the point, though, evolution makes no sense in time frames much shorter than a million years, and significant levels of change can take much longer.

Well, to come to the matter in hand. About 20 fossil species have so far been named in the Hominid line—those creatures purportedly descended, on the human side of the tree, from the common ancestor of Homo sapiens (us) and the two Chimpanzee species of the genus Pan. There were most likely many more. The species that we find today in every genus or family represent, in metaphor, the tips of the twigs of a dense bush. In the case of the little branch that began with the proto-Chimp-Human, only three twig tips remain. In the six or seven million years that have elapsed since that primeval split, who knows how many twigs on the Chimp side, and how many on the Human side, really existed? Several proto-Chimps are known (or suspected), and, as I said, about 20 proto-Humans.

In a book published in 1998, Becoming Human: Evolution and Human Uniqueness, author Ian Tattersall describes what is known of the major changes that arose among the five or six of those proto-Human species that are actually ancestral to Cro-Magnon, the earliest expression of Homo sapiens, which arose between 150,000 and 200,000 years ago. Dr. Tattersall set himself two challenges: to explain to a popular audience the actual bushiness of pre-Human evolution, and to describe and account for the qualitative differences between humans and the rest of the apes.

It has become fashionable in the past generation or so to "demote" humanity from the pedestal on which our cultural habits have placed us. In the author's mind, these demotions have gone too far (Am I becoming a curmudgeon? It seems to me that "going too far" is what people do best, and most frequently, and almost without exception!). Thus, he skates a careful line in this book. The origin of humanity has not been some unrelenting upward march. On the other hand, something very striking has indeed happened, that produced a species that isn't just a big-brained ape, but something new.

To cut to the chase (I'll leave it to you to read this very well written and thoroughly fascinating book), the Cro-Magnons and their cousins the Neanderthals had brains of about equal size. They co-existed, usually not in the same areas, for a few tens of thousands of years. Here is a point that struck me with particular force: Many, but not all, Neanderthal skulls have an arch in the skull base atop the vocal tract, something that is necessary for articulate speech. All Cro-Magnon skulls have it. Yet prior to about 40,000 or perhaps 50,000 years ago, neither species left fossil or archaeological remains that indicate they used symbolic thought. The Cro-Magnon remains of this period are usually termed "anatomically modern". Then, almost overnight, a dramatic cultural shift occurred. Some time in the "thirties" (a little before 30,000 years ago), art of all kinds began to be produced. Cave paintings are rightly famous, but personal artifacts became much more artful, both decorations on useful items and items that are purposely artistic but otherwise (probably) useless. In this period, the term used is "culturally modern."

It is hard to draw conclusions about what actually happened when we can't go back and interview them. We don't even have really good DNA from anybody who lived 30- to 50,000 years ago. Yes, we have enough Neanderthal DNA—all discovered since the book was written—to allow us to determine that various human populations carry between 1% and 6% of Neanderthal DNA. But, as the author describes, skills such as language arose only after the physical (including to the brain) changes needed had arisen for other reasons. These are called exaptations, a kind of adaptation that gets co-opted for another use. Personally, I find it hard to think that Broca's and Wernicke's areas in our left brain hemisphere could have arisen for purposes that we can't discern, and were then found to be just what was needed to support language. In any case, positing pre-existing exaptations, the further changes needed to produce language, symbolic reasoning, and art seem to have taken place quickly, without making any discernible changes in fossil remains. And that is where Dr. Tattersall has to leave it. We still don't know enough to say how such a significant cultural shift occurred.

In the last few pages, he allows himself to speculate on future evolution of Homo sapiens. He thinks not much will happen. I intended earlier to write of the tension between uniformitarianism and catastrophism, both in geology and in biology, but the writing took a life of its own. Let me instead briefly say, a mix of uniformitarian periods and catastrophic events have happened to mold the Earth we see today, and the styles of living species we discover all around us. He points out that species don't evolve much. They go on for millions of years unchanged. But when some circumstance isolates a smallish population of a species, the narrower gene pool available among them is more prone to genetic drift and to the fixation of new, beneficial (even though small) mutations that would be swamped and eliminated from a larger gene pool. Given enough time, usually at least tens to hundreds of thousands of years in the case of large primates, the small "drifted" population will have changed enough that interbreeding is no possible with the larger population from which they arose, should they come back into contact. They are a new species. If they are viable, and even more, better able to thrive in the same environment, they original species may die out in their favor.

Today, what opportunity is there for some small but viable population of humans—say, a few thousand—to remain isolated for 100 or 1,000 centuries? Zero, Zip, Nil, Nada. Small evolutionary changes will continue: maybe our descendants will have fewer back problems; maybe few or none will have wisdom teeth. Our inefficient retina will probably never be replaced with the more efficient squid retina, unless genetic engineering really, really takes off (but would there somehow be a tendency for these engineered folks to spit ink?). We are the first truly global species. That will last as long as we do, or until we experience, or even cause, a catastrophe that reduces humanity to a scattering of small tribes, no more than 100,000 people in groups seldom exceeding a few hundred. Then speciation is possible. Not before. You don't like catastrophes? Then take care of the things that remain. Also, look around. The sorts of people we see today are about what we can expect for the next thousand centuries or so.

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