MODERN TIMES
by Art Hobson
ahobson@uark.edu
NWA Times 27 May 2006
NEW DEVELOPMENTS
IN BIOLOGICAL EVOLUTION
The story of life on Earth is the most fundamental of all histories, an inspiring and epic tale that all of us should understand. We ignore this history at our peril. Without this long view, we can neither understand where we are today nor find a rational and moral path to the future. Unfortunately, Arkansas public schools often ignore it deliberately; a poll of Arkansas biology teachers shows that, due to community pressure, 60 percent of them delete evolution or teach creationism.
Here are four new developments. The first involves "Lucy," a fossil of the species Australopithecus afarensis, the famous direct human ancestor that ranged across east Africa during 3.6-3.0 million years ago. Despite many apelike features, its legs, built for upright walking, put it clearly in the human camp. Now archaeologists have found fossils, from strata directly beneath the rock that held Lucy, of a new 4 million-year-old human ancestor called Australopithecus anamensis. From this creature's remains, it's clear that it evolved some half-million years later into A. afarensis.
At a nearby site, a jawbone of A. anamensis was found 80 meters above a 4.4-million-year-old layer containing fossils of a second new species. Its teeth and skull suggest that it evolved into A. anamensis. The three species are a nice example of the sequential evolution of our human ancestors. These species were remarkably successful, each one thriving for some 500,000 years, 5 times longer than modern Homo sapiens.
The second development concerns how humans and the ancestors of chimpanzees parted ways. Studies of the DNA (the microscopic genetic material) of chimps and humans have yielded new information about ancient evolutionary changes, because DNA evolves over time at a fairly predictable rate.
Recent analysis shows that human ancestors and chimp ancestors first separated some 6.3 million years ago, but that the two "species" then continued interbreeding (the two species would have looked quite similar) for perhaps a million years before finally parting ways. I put "species" in quotes here, because a species is usually defined as a population that cannot produce fertile offspring with other populations. However, in the case of human and chimp ancestors, the hybridization produced infertile males but fertile females. These females could then mate with ancestral chimp males to produce fertile males and with them the hybrid line would be able to reproduce on its own, with results that are still imbedded in our own DNA.
In the third development, scientists discovered a 375 million-year-old fossil fish with a crocodile-like head that could move on land on flippers, the way a seal moves. It's one of many transitional forms that have shown up in the fossil record. 10 jawbones, three skulls, and two fairly complete specimens have been excavated so far. The animal's head is much like the heads of other early four-footed animals, while its body has scales and overlapping ribs, a new development that was needed to support the body outside of water. The chest fins have bones similar to those in an arm. This animal's ability to flop out of the water gave it a competitive advantage in escaping predatory fish, and paved the way for the land animals that followed, including ourselves.
Finally, new research shows how biologically complex structures evolve by reusing and modifying existing parts. "Complex structures" are structures having two or more parts, where any single part is useless without all the other parts. Since evolution proceeds by single genetic changes, its hard to understand how all the parts of a complex structure could evolve because some parts would evolve before others but all parts are needed simultaneously for proper functioning. Although there's never been any controversy among scientists over whether these systems evolved, there has been controversy over the details of how this evolution occurred. Creationists have recently latched onto this controversy to try to dispute evolution itself, stating that such "irreducibly complex" systems must have been miraculously created whole.
One example of biological complexity is hormones and hormone receptors, molecules that act like pairs of locks and keys. Hormones fit into or "bind with" specific receptors, and this connection sends a signal to turn cell functions on or off. Since either the lock or the key is useless by itself, how could both parts have evolved?
Scientists have now found the answer. They studied the evolution of a certain class of hormone receptors in fish, and found from genetic analysis that they all descended from a single common receptor 450 million years ago. This single receptor could bind not only with its modern hormone (call it hormone A), but also with a second hormone. It turns out that hormone A didn't exist 450 million years ago, so that at that time the receptor could bind only with the second hormone. Tens of millions of years later, A-hormone evolved to fit the same receptor. A genetic mutation then caused the single receptor to mutate into two varieties, one that could bind only to hormone A and a second that could bind only to the second hormone. This historical example indicates that complex systems such as hormone A and its receptor can evolve by exploiting older molecules that had been previously used in a different role.
Stay tuned. Science is
never a closed book, for its conclusions are based on evidence, and new evidence
can contradict old conclusions and lead to a better understanding of nature. Anytime anybody tells you that they are
"absolutely certain" of some general idea, you can be assured that
their conclusion is not scientific, because science never produces absolute
certainty.