8.18–8.22: The evidence for evolution is overwhelming.

The Denise’s Pygmy seahorse (Hippocampus denise) hides itself in a sea fan.

In the 150 or so years since Darwin first published The Origin of Species, thousands of studies have been conducted on his theory of evolution by natural selection, both in the laboratory and in natural habitats. A wide range of modern methodologies has also been developed—such as genome sequencing and algorithms for comparing genome similarities and constructing the most likely phylogenies—all contributing to a much deeper understanding of the process of evolution. This ongoing accumulation of evidence overwhelmingly supports the basic premise that Darwin put forward, while filling in many of the gaps that frustrated him.

In the remainder of the chapter we review the five primary lines of evidence demonstrating the occurrence of evolution:

• 1. The fossil record—physical evidence of organisms that lived in the past
• 2. Biogeography—patterns in the geographic distribution of living organisms
• 3. Comparative anatomy and embryology—growth, development, and body structures of major groups of organisms
• 4. Molecular biology—examination of life at the level of individual molecules
• 5. Laboratory and field studies—implementation of the scientific method to observe and study evolutionary mechanisms

The first of the five lines of evidence is the fossil record. Although it has been central to much documentation of the occurrence of evolution, it is a very incomplete record. After all, the soft parts of an organism tend to decay rapidly and completely after death. And there are only a few environments (such as tree resin, tar pits, the bottom of deep lakes, and continental shelves) in which the processes of erosion and decomposition are so reduced that an organism’s hard parts, including bones, teeth, and shells, can be preserved for thousands or even millions of years. These remains, called fossils, can be used to reconstruct what organisms must have looked like long ago. Such reconstructions often provide a clear record of evolutionary change, including unique insights into the patterns and processes of history (FIGURE 8-32).

Figure 8.32: Evidence for evolution: the fossil record. Fossils can be used to reconstruct the appearance of organisms that lived long ago.

The use of radiometric dating helps in painting a clearer picture of organisms’ evolutionary history by telling us the age of the rock in which a fossil is found (FIGURE 8-33). In Darwin’s time, it was assumed that the deeper in the earth a fossil was found, the older it was. Radiometric dating goes a step farther, making it possible to determine not just the relative age of fossils but also their absolute, numerical age. This is accomplished by evaluating the amounts of certain radioactive isotopes present in the fossil-containing rocks or the layers above and below them. Radioactive isotopes in a rock begin breaking down into more stable compounds as soon as the rock is formed, and they do so at a constant rate. Nothing can alter this. By measuring the relative amounts of the radioactive isotope and its leftover decay product in the rock where a fossil is found, the age of the rock, and thus of the fossil, can be calculated.

Figure 8.33: How old is that fossil? Radiometric dating using uranium-238 helps to determine the age of rocks and the fossils in them.

Radiometric dating confirms that the earth is very old. Rocks more than 3.8 billion years old have been found on all of the earth’s continents, with the oldest so far found in northwestern Canada. By using the radioactive isotope uranium-238, with a half-life of 4.5 billion years, researchers have determined that the earth is about 4.6 billion years old and that the earliest organisms appeared at least 3.5 billion years ago. Radiometric dating also makes it possible to put the fossil record in chronological order. By dating all the fossils discovered in one locale, paleontologists can better evaluate whether the organisms are related to each other and how groups of organisms changed over time.

Paleontologists must deal with the fact, however, that fossilization is an exceedingly unlikely event, and when it does occur, it represents only those organisms that (1) happened to live in that particular area, (2) could be preserved under certain chemical conditions, and (3) had physical structures that can leave fossils. For this reason, the fossil record is unavoidably incomplete. Entire groups of organisms have left no fossil record at all, and for others the record has numerous gaps. Still, for what there is, the fossil record can be incredibly detailed and interesting, and fossils have been found that link all of the major groups of vertebrates.

The evolutionary history of horses is among the most well-preserved in the fossil record. First appearing in North America about 55 million years ago, horses radiated around the world, with more recent fossils appearing in Eurasia and Africa. These fossils exhibit distinct adaptations to those differing environments. Later, about 1.5 million years ago, much of the horse diversity—including all North American horse species—disappeared, leaving only a single remaining genus, or group of species, called Equus. Because there is now only one horse genus on earth, it is tempting to imagine a simple linear path from modern horses straight back through their 55-million-year evolutionary history. But that’s just not how evolution works. In reality, there have been numerous branches of horses that have split off over evolutionary time, flourished for millions of years, and only recently gone extinct. What we see living today is just a single branch of a greatly branched evolutionary tree (FIGURE 8-34).

Figure 8.34: An evolutionary family tree. The branching evolutionary tree of the horse.

The fossil record provides another valuable piece of evidence for evolution, in the form of fossils with transitional features. These are fossils that demonstrate a link between groups of species thought to have shared a common ancestor. One such fossil is Tiktaalik (FIGURE 8-35). First found in northern Canada and estimated to be 375 million years old, Tiktaalik fossils seem to represent a transitional phase between lobe-finned fishes and terrestrial vertebrates. These creatures, like lobe-finned fishes, had gills, scales, and fins, but they also had arm-like joints in their fins and could support their bodies with their limbs in much the same way that salamanders can.

Figure 8.35: A missing link?