Evolution: a paleontologist's perspective

by Dr. Niles Eldredge

Evolution is the only scientific concept that explains the history and diversity of life. Darwin was probably the first to pose the question that made his famous theory testable: If evolution is true—if life really has evolved—what would I expect to observe as a consequence? In other words, how could I test this concept?

Two of the most important general tests of the notion of evolution are:

These two predictions have been continually tested ever since Darwin. (Indeed, they were part of the evidence that led Darwin and others to their evolutionary views in the first place.) These predictions have been confirmed over and over again. Together with natural selection, speciation, and other evolutionary mechanisms, evolutionary theory is one of the most important and powerful scientific theories in the history of ideas.

The fossil record bears on both of these predictions. All extinct forms of life fit into the nested patterns of resemblance that form the core of the tree of life. For example, the trilobites I study are extinct members of the phylum Arthropoda, which includes living crustaceans, arachnids, insects, and other groups. Trilobites share with all these other groups a segmented external skeleton and compound eyes; however, they are so primitive within Arthropoda that experts disagree about whether they are more closely related to Crustacea or Arachnida, which include the trilobite-like horseshoe crabs. And there has indeed been a sequence from simpler forms of life up through the more complex animals and plants, abundantly and consistently confirming prediction 2, as the fossil record tells us loudly and clearly.

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The Earth was formed circa 4.65 billion years ago. The first evidence of life consists of fossils of organisms similar to today's blue-green algae and other forms of bacteria that are at least 3.7 billion years old—though chemical traces in still older sedimentary rocks suggest that life was present even earlier. Bacteria are the most primitive forms of life known; their cells contain DNA but lack the nucleus that houses DNA in complex eukaryotic cells such as our own.

These eukaryotic cells—somewhat larger, but still microscopic—appeared circa 2.5 billion years ago. Their emergence was correlated with a rise in atmospheric oxygen that some scientists think was essential to this evolutionary step. Ciliates, flagellates, and many other groups of single-celled organisms (some rather plantlike, others more animal- or fungus-like) are the simplest, most primitive members of the Eukaryota. The other eukaryotes—true multicellular plants, fungi, and animals—appeared later, with red algae entering the fossil record 1.2 billion years ago.

The next multicellular life forms to appear in the fossil record are the Vendian Period (also called the Ediacaran period) faunas, which are approximately 600 million years old. Paleontologists have argued over the affinities of these organisms. Some interpret them to be a novel evolutionary experiment that produced an array of organisms not closely related to today's multicellular plants, fungi, and animals. Others, however, see in these fossils evidence for cells of a single type function together to accomplish a task—what biologists call "tissue-grade" organization. This organization is commonly seen in members of groups such as Cnidaria (corals and jellyfishes) and Ctenophora (comb jellies), which are primitive forms of animal life still very much with us today. Their bodies consist solely of layers of tissues that perform all their physiological functions, such as digestion; these organisms do not have true organs like stomachs. Some paleontologists interpret fossils such as Pteridinium, an Ediacaran organism, to be early members of the Cnidaria. Thus, it seems likely that the more primitive forms of animal life do in fact precede more complex forms.

The fossil record grew considerably more complicated when the Cambrian Explosion began circa 542 million years ago. Within roughly 10 million years, all of the most complex forms of animal life appeared, including the phyla Arthropoda, Mollusca, and Echinodermata (starfish, sea urchins, etc.). Arthropods and mollusks are protostomes; during their embryological development, the first opening formed, the blastopore, becomes the mouth. Echinoderms are deuterostomes, meaning that the mouth develops from a second hole formed in the embryo. Vertebrates are also deuterstomes.

The vertebrate fossil record goes on to illustrate a sequence of forms ranging from primitive to more derived. Jawless fishes precede jawed fishes; bony fishes precede amphibians; amphibians precede reptiles; and reptiles precede both birds and mammals. The fossil record of human evolution reveals the same predicted pattern: a progression of smaller brains to larger ones. The most ancient hominids had brains roughly the size of modern chimpanzees, while the biggest-brained species (Neanderthals and ourselves) came latest in the sequence.

Doubters of evolution claim that the fossil record does not yield forms intermediate between groups. On the contrary, the origin of limbed tetrapods from fish, the origin of mammals from mammal-like reptiles, and the origin of birds from terrestrial dinosaurs—all have rich and dense fossil records showing the pathways of evolution from earlier to later forms.