Thursday, June 2, 2011
Evolutionary Transitions: Still the Key to Understanding
When I first taught Evolutionary Biology in the West Indies more than 4 decades ago, I drew the attention of a Scripture teacher passing by my classroom. He'd evidently peeked in and spotted the large 24" by 36" diagram on the wall showing various stages of human evolution, from Australopithecus 3.5 million years ago, to Homo Sapiens today. He poked his head into the class briefly and inquired: "Do you really, seriously believe that this chart shows how we originated? That it portrays how we came to be here?" I replied: "Sure! And if I didn't believe it I certainly wouldn't be teaching it!".
But the issue of evolutionary transitions almost always incites problems for many people, not only those who've never taken a college (or even high school) biology course, but even those who have but can't wrap their heads around the concepts, or principles. Often I have found they get easily misled by what I call 'macguffins': issues or side shows not critical to evolution but to which the media seems to allot much attention or importance.
Most people, for example, aren't aware that evolution theory is not one monolithic framework but incorporates algorithmic components that can lead to differing interpretations of transitional species: 1) gradualism, 2) punctuated equilibrium, and 3) mixed transitions including both (1) and (2). While a "missing link" (exact common ancestor identified by specific genome at a specific time) would be more or less standard for the gradualist view it is not needed for the latter two. We are able to see more in terms of algorithmic branching in the latter, and how numerous competitor common ancestors could have emerged rapidly and co-existed. (Just as we now have evidence that both Neandertal Man and modern Homo Sapiens existed at the same time, for perhaps 50,000 yrs.)
A sound response to such questions on "missing links" is always for the evolutionist to refer directly to the evidence for a "genetic missing link" which would cover both the gradualist and punctuated equilibrium idioms. Specifically, in one of the most powerful ever demonstrations of the validity of evolution, Yunis and Prakash, 1982, Science, Vol. 215, p. 1525, 'The Origin of Man: A Chromosomal Pictorial Legacy', showed that the human chromosome designated '2' was the result of the telomeric fusion of the two ape chromosomes, 2p and 2q. The effect also saw the reduction from 24 chromosome pairs in apes, to 23 pairs in humans. In other words, the duo of ape chromosomes (2p and 2q) can be considered a genetic missing link to humans!
Less well known, if that can be believed, is the crucial role that dentition analysis and tool making play in sorting the fossils of prehistoric humans. For example, one of the first questions the investigator will ask is whether a given jaw and teeth found in it, can accommodate flesh eating. For some fortunate cases, this is also answered by the fossils found in the vicinity of the hominid ones. For example, in the case of one Au. garhi fossil (see image of this hominid) an antelope jawbone was found nearby and on it ancient cut marks disclosing its tongue had been sliced out using a stone tool. Radio-nuclide dating of both fossils traced them to the same time. Clearly, Au. garhi had a taste for antelope tongue much like his more distant cousins.
In more ways than one, paleoanthropology is an adjunct to evolution proper, as well as microbiology. Together they give a very complete and uniform picture of how huans evolved over the past 4-5 million years. We can glean similarly complete picture from studies of the intermediary or transitional fossils of other species, such as the elephant (see transitional imagery).
What about alleged fossil problems associated with evolution, or as some critics aver, “gradualism” (given we now use this word only casually and not formally!) Well, as Richard Dawkins has observed we do know that most of the major invertebrate groups seem to suddenly appear in the Cambrian rock strata of 600 million years ago as if “planted” (The Blind Watchmaker, p. 229) Evolutionists concede this leaves a large hole or gap in the fossil record, but one which is not without logical explanation! Consider the backward time trajectory within which we see fossils by older layers (dated using radioactive decay – as I showed in a prior blog) accumulating the further back we go. We see a bunch of invertebrates at 600 my ago but virtually nothing before. Why? A logical explanation is that even more primitive forms pre-existed these and their soft exo-skeleta or body parts would simply not have survived. In other words there were no shells or bones to fossilize! Think of just a large primitive jellyfish, would we find fossil evidence for it? Hardly! The creature is 100% soft tissue none of which would have been preserved. Same with most other more primitive denizens. Again, this explanation is accepted by BOTH punctualists and gradualists
Another reason for apparent “jerks” in the fossil record is offered by Prof. Dawkins (op. cit., p. 240)
“The reason that the transition from ancestral species to descendant species appears to be jerky is simply that, when we look at a series of fossils from any one place, we are probably not looking at an evolutionary event at all, but a migrational event. The arrival of a new species from another geographical area”
This actually is perhaps the simplest and most logical explanation of all. I mean, there’s no law that commands all species remain fixed in one place! Even today, as water holes dry up or food reserves dip, African animals like Wildebeest, antelope and others will migrate to new areas in search of resources to support them.
Not mentioned by Dawkins is how many other ‘holes” or gaps may have been created by virtue of tectonic plates (which make up the Earth’s crust) slipping or crushing key fossils in distinct layers. (Or, totally destroying the layers and embedded fossils.) Obviously, if such tectonic traversal displaced a fossil layer, then it likely won’t be found when it may well have harbored intermediate forms. Thus, in effect, "absence of evidence is not evidence of absence"!
The bottom line, as of this writing, is we are very confident of the transitional record assembled so far, not just for human evolution but for evolution of hundreds of other species as well - from Trilobites to elephants. Sure, there will always be naysayers - there are in every field- but until they can come up with their own unique theory (which must include its own falsification) they are merely wasting their time...and ours!