December 2006

Was there a concept of evolution before Charles Darwin?

Dorudon atrox, an ancestral whale from the Late Eocene of Egypt. Pencil drawing, digital coloring by Arthur Weasley.

Gingerich: Keith Thomson’s book Before Darwin: Reconciling God and Nature gives an excellent history of the concept of evolution before Darwin arrived on the scene. He does a marvelous job of outlining how much was known about evolution before the 1850’s. It’s fascinating to read. Darwin didn’t invent evolution. Darwin developed the current and viable explanation of how it works as a process. Evolution was well established in the decades and even a century before Darwin in the sense of knowing that life changed through time.

What does the fossil record look like on the microevolutionary scale [within a species]?

Gingerich: It is one of continuity and discontinuity. We know that some species don’t change much over time, others exhibit changes, some get bigger or smaller, and so on. We see lineages appear suddenly in the fossil record—ones that we can’t explain—while in other cases, early primates for instance, we have been able to trace successive species through time. So there are many different patterns in microevolution.

Is this pattern the same or different on the macroevolutionary scale [at or above the species level]?

Gingerich: Macroevolution and microevolution are parts of a continuum that are distinguished more by the scale of time on which they are studied. Macroevolution, generally speaking, is what paleontologists study on time scales of thousands to millions of generations. Macroevolution is evolution that happens on a grand time scale and explores questions such as the origin of major groups of plants and animals, and the development of novel innovations like sexual reproduction. Microevolution is what people can study in laboratories or in the field from a few up to a thousand generations. The evolutionary process itself, though, is not even microevolutionary—the process takes place on a generation-to-generation time scale.

Are you saying that the rate of evolution is fast?

Gingerich: Evolution takes place on short time scales, from one generation to the next. When you study evolution, macroevolution or microevolution, over many generations, it is often slow, but when you study evolution on the time scale of the process, a generation at a time, the change you can measure is generally fast. An example would be the evolutionary history of the horse, whose history starts in the early part of the Eocene. We have hundreds and hundreds of fossils that trace its early history. It appeared the size of a Siamese cat, then it was replaced by an animal the size of a fox, then one the size of a coyote—in incremental steps—until it reached today’s size. These changes were slow, but when you measure the difference between successive fossil samples, you see that evolution was faster on shorter time scales. This is what happens when morphology is constrained and time [geological time] is long.

Evolution is a much more dynamic process than most people think. If you study it on short time scales, it’s very fast. It doesn’t take millions of years to make new structures or to adapt to new conditions. It takes a few generations—not even hundreds or thousands of generations. It’s well known, for example, that students have gotten taller during the last few human generations. I think I can see this in the students I have taught for the past generation. Anthropologists have studied human stature and call this increase over time the secular trend in human physical growth. This means the fossil record is more the record of the environmental conditions during which change took place than it is a record of the evolutionary process itself. This is why the fossil record looks punctuated, that is, exhibiting periods of intense evolution and periods when nothing much seems to happen.

How do you calculate the rate of evolution?

Gingerich: In mathematical terms, a rate of evolution depends not so much on the numerator of the ratio but rather on the denominator. A rate is a ratio with a numerator [in this case, change] and denominator [time]. If you study a long-term fossil record, you will see slow rates because that’s all you can see on this macroevolutionary time scale. But on a microevolutionary scale, where the time frame is shorter, rates are systematically faster. When you do lab experiments, they are faster still. When you plot all of these time scales together, you see a perfectly continuous distribution. You can then examine evolution on a generation-to-generation time scale; evidence shows it’s fast. It doesn’t mean evolution cannot be slower, but the upper limit is very fast.

Knowing the rate of evolution is important because rates:

• quantify, or measure, evolutionary change in relation to time, and
• indicate how the process of evolution works.

Some scientists use a rate in darwins, which uses a standard of one million years. I prefer to use a rate using haldanes, which uses a standard of one generation. Both calculate rates in terms of proportional change divided by elapsed time. What we are really interested in, from the point of view of the process, is generation-to-generation rates or rates calculated on one-generation time scales.

Philip Gingerich working on a fossil whale in the field in Egypt.
Photo by Jeffrey A. Wilson

Your team was the first to find skeletons linking whales to land mammals. Does the fossil record to date indicate a rapid change from land to sea?

Gingerich: Whales have not been collected on a fine enough time scale to see rapid change. This will be revealed through more fieldwork. So far we have fossils illustrating three or four steps that bridge the precursor of whales to today’s mammals.

Fossils document biological change through geological time. The fossil record, of course, is supported by molecular and other studies showing that whales share a common ancestor with four-footed, hoofed mammals such as cows and hippos. Their evolution is particularly interesting because early vertebrates came from the sea to live on land, and whales then returned to an aquatic life.

My research focuses on archaeocetes, or “archaic” whales that were the ones that evolved from land. We find whale fossils from the Eocene epoch, which lasted from about 54.8 to 33.7 million years ago [mya]. These include:

• complete skeletons of middle-to-late Eocene Basilosauridae (e.g., Dorudon and Basilosaurus) that were the first known to retain hand limbs, feet, and toes
• exceptionally complete skeletons of middle Eocene Protocetidae (e.g., Rodhocetus and Artiocetus) that connect whales to an artiodactyl ancestry
• a partial skull of earliest middle Eocene Pakicetidae (the Pakicetus) that was at the time the first skull of the oldest known whale

What are some of the key discoveries about whale history?

Gingerich: The oldest whale fossil, Himalayacetus, was found in India in Eocene marine strata, indicating it was about 53 million years old. It has the misfortune of being represented only by a lower jaw with two teeth in it. It shows one interesting characteristic: It doesn’t yet exhibit the enlarged mandibular canal later linked to hearing in water. That happens soon afterward. Another interesting characteristic is that this fossil is found in marine rocks. This puts other whale fossils that were contemporaries, such as those of the riverine Pakicetus, in perspective. All of the earliest whales that we know about so far were semi-aquatic. I am sure that they were still coming on land to give birth, to rest, and to mate, very much like modern sea lions.

Other fossil examples provide additional evidence. For example, nearly complete skeletons of Rodhocetus and Artiocetus from the early middle Eocene represent foot-powered swimmers with large webbed feet. We now have a complete skeleton of Rodhocetus. It’s an important find because it illustrates and allows us to quantify the whale’s transition from land to water. The proportions of Rodhocetus ’ limbs, skull, neck, and thorax indicate it was a foot-powered swimmer. It would take subsequent generations to evolve into tail-powered swimmers. By the mid- to late Eocene, ancient whales such as the Dorudon were swimming like the whales of today, using their tail.

At the close of the Eocene, or early in the next epoch, the Oligocene [33.7 to 23.8 mya], the archaic whale lineage began to divide into two groups leading to the toothed whales and the baleen whales, and these in turn evolved into the wonderful whale diversity we see today.

What have we learned so far from whale fossils?

Gingerich: Whale fossil finds enable us to document the evolutionary history of whales, a history we were postulating from theory before:

• Whales are warm-blooded mammals that evolved backwards, from land to sea, which shows that evolution can go both ways; it is opportunistic, not deterministic.
• It hasn’t been a smooth transition for whales. There is a stage between specialized foot-powered swimmers like Rodhocetus and modern whales: the stage of tail-powered swimmers like Dorudon that still retain vestigial hind limbs.
• Modern whales that are carnivorous today evolved from ancient artiodactyls [the mammalian order including cows, deer, hippos, etc.] that were plant eaters. It’s an interesting change in feeding strategy, from eating plants to eating animals.

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