What evidence is there for evolution?

Paleontology is simply the study of prehistoric life. It studies the evolutionary history of organisms, along with how past organisms interacted with each other and their surroundings. The work of paleontology is to improve our understanding of past organisms. By one estimate, the number of species that ever lived is 10 billion, so paleontologists have a lot of work to do.

Fossils are a significant source of evidence for evolution. Here's some of the evidence we see:(1)

• Fossils in geologically young strata are similar or identical to existing organisms.
• The older the strata, the more the fossils differ from today's organisms.
• Gaps in the fossil record have been gradually filled in as more discoveries are made.
• Transitional fossils that are found largely have the features that evolution predicted they would have.
• Fossils of any species are only found in strata younger than their ancestor species. “If we found a rabbit in strata from the Jurassic era (206-144 Mya), as the great evolutionary biologist J.B.S. Haldane, once remarked, we would consider abandoning the theory of evolution.”(2)

Putting this all together, we see life evolving over four billion years. We've found abundant examples of transitional fossils showing features that are part-way between earlier fossils and later ones. With an estimated 10 billion organisms to try to track, we will never know every detail of evolutionary history. Fortunately for researchers in paleontology, there will always be unanswered questions to answer.

Biogeography is the study of how organisms vary across space and time. The Earth's crust is constantly changing as continental and oceanic plates move. Mountains rise and fall, and oceans form and disappear. These changes greatly affect the distribution of life. In fact, we can read the story of evolution from where organisms have been and where they are now.

Biogeography is a significant source of evidence for evolution. Here is some of the evidence we see:

• Similiar species can be traced to a place of origin. Species originate in one place and spread out from there. If we find similar species in Europe and North America, we expect to be able to trace similar species back in time to a point where they shared a common place. This is exactly what we see. For example, the animals of Europe and North America are similar, and geological evidence reveals that Europe and North America were once connected.
• When a barrier splits a population of organisms, the species on each side of the barrier gradually become more different. The longer a barrier has been in place, the more differences we should find between the species. This is exactly what we see. For example, Europe and North America were connected 40 million years ago (Mya), while Africa and South America were connected 80 Mya. We expect more similarity between the animals of Europe and North America than we do for the animals of Africa and South America, and this is what we see.(3)

The finches of the Galapagos Islands are a classic example of evidence for evolution from biogeography. The different islands have different species of finches. None of these species appears on nearby South America. Yet the species of finch most like those on the island are found in South America. This is evidence of common ancestry because similar species appeared close together (the islands and mainland South America), and yet the species are different from island to island (separated by an ocean barrier).

Developmental biology studies how organisms develop into mature adults from single cells. One especially well-studied area is how vertebrate embryos develop. They pass through various stages that are remarkably similar. All vertebrates have certain common features, some of which are seen only in the embryo stage in a lot of the animals. There are four main features shared by all chordates—the large group that includes vertebrates—which are not found in other groups of animals. These are the following:

• the presence of a rod-like structure called the notochord;
• a dorsal, hollow nerve cord;
• a post-anal tail; and
• pharyngeal pouches or slits, sometimes referred to as pharyngeal gill slits.

Being vertebrates we have all of these features at some time during our development. What is interesting is the great differences in what the final result turns out to be. Here are some examples:

• There are some small chordates called lancelets that still have a notochord even as adults. It gives rigidity to their bodies. In us the notochord present in early stages is seen only as remnants that form the disks in our spinal column.
• In fish, the pharyngeal structures become supports for their gills. In us these same structures form a portion of our inner ear that we use for hearing things.
• Vertebrates have five digits in their very early stage of development. In cattle and pigs, these get reduced and fused to produce a cloven hoof. In horses they reduce to a single-digit hoof. They become wings in birds, and in us they become hands or feet.

These developmental patterns are evidence of evolution because they make sense as the features of common ancestors that have changed gradually over time. We can produce rough evolutionary histories from what we learn from developmental biology, and these histories agree with the other lines of evidence.

Morphology, in biology, is an organism's shape and structure. We can group organisms together according to their morphology. For example, we put organisms with six legs in the group Insecta and those with eight in the group Arachnida. We can further classify organisms by whether or not they have membranous wings. This happens to divide Insecta into smaller groups. If we keep doing this, we end up with a hierarchy of groups within groups.

This is interesting because it need not have been the case. It could have been that organisms could only be grouped by the first common feature they share. Grouping for the next common feature might have required starting over with new groups. For example, if there were arachnids with membranous wings, we wouldn't have been able to just divide Insecta. We would have had to ditch the Insecta and Archnida groups and start the hierarchy all over again.

If we ask why things are this way, evolution offers a very simple answer. Evolution explains this as organisms inheriting traits from their ancestors. Bats and bird both have wings, but their wings have different structures, so the wings didn't come from the same ancestor. However, the bones in bat and bird wings are just different arrangements of similar bones found in human arms, dog legs, and horse legs. Evolution explains all this very simply. All these animals share a common ancester that had all these bones. Such similarites are called homologies.

Morphology offers another source evidence for evolution. We see organisms having different structures that serve the same purpose. Some of these structures are more efficient than others. Why wouldn't all organisms use the most efficient structure?

Take, for example, the eye. In vertebrates, the photoreceptors have to reflect off of the back of the eye. This gives vertebrates a blind spot in their vision. In an octopus eye, the photoreceptors receive light directly, so there is no blind spot. Evolution explains why not all organisms have the eye of an octopus: life is constrained by ancestry.

Genetics is the study of DNA and how DNA affects organisms. This includes, of course, genes. Did you know that the more similar two animals look on the outside, the more similar their genes are? An objective observer can easily see that humans are more similar to orangutans, less so to rats, and even less similar to daisies. What we find is that the number of genes we have in common with those organisms correlates with your observations.

As with the preceding lines of evidence, we can yet again group organisms into a hierarchy according to how similar their genes are. There are so many genes that it's easy to imagine that this would be hard to do. It could have been that each time you group according to a new set of genes you're forced to start over with a new hierarchy.

Let's compare how similar human genes are to the genes of other animals, restricting ourselves to genes that produce proteins.

• Yeast - 45% identical
• Fruit flies - 60% identical
• Chickens - 90% identical
• Rabbits - 95% identical
• Chimpanzees - 98% identical!

This is why fruit flies make excellent study subjects for research in human genetics. We can learn about 60% of our genetic information by studying a species that is plentiful, cheap, easy to rear, and prolific.

We have a simple explanation for how all this happened. It's called evolution, and the contingencies of evolution explain a lot of the oddities we find in genetics.

Evolution predicts that we should find fossils of fish with legs. Well, guess what! We do find those fossils. Lots of them. They show a gradual change from fish to four-legged terrestrial animals, or tetrapods. You might call them fishibians or fishapods.

It all starts with ancient lobe-finned fish, a group that includes today's lungfish and coelacanths. Lobe-finned fish have robust bones and muscles like tetrapods, unlike regular fish. They also move their fins in a “step-cycle” like tetrapods, also unlike regular fish.

Many kinds of fish today crawl out of the water using their fins, like the mudskipper and the walking catfish, so it's not surprising that we find fossils of lobe-finned fish that could do the same.

• Panderichthys was an ancient fish that may not yet have been able to walk, but it was already starting to look like a tetrapod. It had lost the dorsal fin and had four fins where tetrapods have four legs.
• Tiktaalik was a tetrapod-like fish that could drag itself over the ground. This fish had wrists in its robust front fin-feet.
• Acanthostega and Ichthyostega occur in still later strata. These four-legged fish could crawl on all four legs but still had gills and tail fins for swimming.

The hippopotamus has more DNA in common with whales and dolphins—with cetaceans—than with any other animal alive. This suggests that cetaceans are the hippos' closest living relatives. Curiously, both animals can hold their breath for a long time when swimming under water, only periodically coming up for air. It turns out that the fossils of both animals can be traced to early hooved mammals found only near or in water. Yes, whales descended from hooved mammals that lived on land! We call this group the Cetartiodactyla.

Here are some of the transitional fossils we have for cetaceans:

Although cetaceans no longer have legs, they still have some leg bones in their bodies. They all still have shrunken pelvises, and the sperm whale still even has shrunken femurs.

Humans did not come from monkeys. We share a common ape-like ancestor instead. That ancestor lived between 5 and 8 million years ago (Mya). The apes we see today are very distant cousins of ours. In fact, out of the entire animal world, our closest cousins are the chimpanzees.

Many lines of evidence confirm that chimpanzees are our evolutionary cousins, including the fact that we share 98% of our DNA. But fossils provide us with one of the more striking confirmations. We see a gradual change from chimp-like features to human-like features over about 5 million years. The easiest change to trace is brain size. Chimp brains are between 300 and 500 cc (cubic centimeters) in volume, so our common ancestor probably had brains that size too. In the following chart you can see how the braincase sizes of hominids—primates with human-like features—gradually increased from chimp-sized to human-sized.

Hominid Species	Age	Braincase Size
Australopithecus afarensis	3.9 - 3 Mya	390-550 cc
Australopithecus africanus	3.5 - 2.5 Mya	400-500 cc
Homo habilis	2.2 - 1.6 Mya	590-690 cc
Homo ergaster	1.9 - 1.6 Mya	700-850 cc
Homo erectus	1.8 - 0.05 Mya	800-1250 cc
Homo heidelbergensis	0.6 - 0.4 Mya	1100-1400 cc
Homo sapiens	< 0.5 Mya	1000-1600 cc

The earliest have chimpanzee features like large canine teeth, thin tooth enamel, elongated skulls, heavy brow ridges, and short legs and long arms. As their brain sizes increase, the canines shrink in size, the tooth enamel thickens, their foreheads move forward, and their limbs become more proportioned like modern humans (Homo sapiens). The heavy, ape-like brow ridge is one of the last features to disappear.

Many organisms have structures that have no apparent use, or that at least don't seem necessary. These are called vestigial structures. Here are some examples:

• Whales and some snakes, like boas and pythons, have remnant hip bones and thigh bones (femurs) inside their bodies.
• Horses have splint bones that have no function, but if one breaks, the horse becomes crippled. (They are remnant side toes.)
• Human appendix, tonsils, tail bones (coccyx), exstrinsic ear muscles (for wiggling ears), and a third eyelid (tiny fold in corner of eye). We still get goose bumps when we're cold, even though we have very little hair to raise to keep us warm.
• Flightless birds like emus and ostriches still have wings, and the kiwi just has as stump where wings might have been.
• Many species that live in caves have eyes that cannot see. There are blind cave fish, salamanders, insects, and spiders. See for example the Texas Blind Salamander, the Comal Springs Dryopid Beetle, and other Texas invertebrates.
• Many species of dandelions have flower petals, pistils, and stamens, but they don't use them. These organs are for sexual reproduction, but many dandelions reproduce asexually.

These same nonfunctional structures actually appear in other animals fully functional, providing a strong clue about their origin. Evolution offers a simple explanation for this state of affairs. Vestigial structures are features that were necessary in ancestors which became unnecessary in their descendants. They are simply remnants of the past. Evolution is likely in the process of removing these structures.

Remember how all of the clues have to point to evolution or else evolution can't be right? Well, one clue seemed at odds with evolution. Humans have 23 chromosome pairs, and chimpanzees have 24 pairs. It's as if some ancestor lost an entire chromosome pair, but no organism can lose an entire chromosome and survive. If evolution is true, then there's only one possible explanation. Two ancestor chromosomes must have merged together into one.

Until recently, this was just an untested hypotheses. After sequencing both the human genome and the chimpanzee genome, it finally became possible to test the hypothesis. Here's what scientists found:

Let's make sense of this. There are special regions on chromosomes called telomeres and centromeres. Telomeres are at the ends of a chromosome. They help keep the chromosome from unraveling, like knots at the end of a rope. The centromere is somewhere between the ends. When a chromosome gets copied, the centromere is where the two copies end up attached before they finally separate. The telomeres and centromeres are different on all the chromosomes. Each chromosome also has unique DNA between them.

So what do we see when we look at the human and chimp chromosomes? Two entire chimp chromosomes are found in one human chromosome—human chromosome 2. Chromosome 2 has four telomeres and two centromeres, and they are identical to the chimp telomeres and centromeres. Chromosome 2 still works because two of the telomeres and one of the centromeres have been turned off. Evolution predicted that we would find this, and we did. This curious clue ended up further confirming evolution.