A Horse Is a Horse, of Course, of Course

This drawing was created in 1848, but it"s likely that you recognize the animal it depicts as a horse. Although horses haven"t changed that much since this drawing was made, they have a long evolutionary history during which they changed significantly. How do we know? The answer lies in the fossil record.

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api/deki/files/18141/Horseevolution.png?revision=1&size=bestfit&width=462&height=561" />Figure (PageIndex2): Evolution of the Horse. The fossil record reveals how horses evolved. The lineage that led to modern horses (Equus) grew taller over time (from the 0.4 m Hyracotherium in early Eocene to the 1.6 m Equus). This lineage also developed longer molar teeth and the degeneration of the outer phalanges on the feet.

Fossils are a window into the past. They provide clear evidence that evolution has occurred. Scientists who find and study fossils are called paleontologists. How do they use fossils to understand the past? Consider the example of the horse, outlined in figure (PageIndex2). Fossils spanning a period of more than 50 million years show how the horse evolved.

The oldest horse fossils show what the earliest horses were like. They were only 0.4 m tall, or about the size of a fox, and they had four long toes. Other evidence shows they lived in wooded marshlands, where they probably ate soft leaves. Over time, the climate became drier, and grasslands slowly replaced the marshes. Later fossils show that horses changed as well.

They became taller, which would help them see predators while they fed in tall grasses. Eventually, they reached a height of about 1.6 m. They evolved a single large toe that eventually became a hoof. This would help them run swiftly and escape predators. Their molars (back teeth) became longer and covered with hard cement. This would allow them to grind tough grasses and grass seeds without wearing out their teeth.

Comparative Anatomy

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Figure (PageIndex3): Mammals (such as cats and whales) have homologous limb structures - with a different overall look but the same bones. Insects (such as praying mantis and water boatman) also have homologous limbs. Cat legs and praying mantis legs are analogous - looking similar but from different evolutionary lineages.

Comparative anatomy is the study of the similarities and differences in the structures of different species. Similar body parts may be homologous structures or analogous structures. Both provide evidence for evolution.

Homologous structures are structures that are similar in related organisms because they were inherited from a common ancestor. These structures may or may not have the same function in the descendants. Figure (PageIndex3) shows the upper appendages of several different mammals. They all have the same basic pattern of bones, although they now have different functions. All of these mammals inherited this basic bone pattern from a common ancestor.

Analogous structures are structures that are similar in unrelated organisms. The structures are similar because they evolved to do the same job, not because they were inherited from a common ancestor. For example, the wings of bats and birds, shown in the figure that follows, look similar on the outside and have the same function. However, wings evolved independently in the two groups of animals. This is apparent when you compare the pattern of bones inside the wings.


Comparative Embryology

Comparative embryology is the study of the similarities and differences in the embryos of different species. Similarities in embryos are likely to be evidence of common ancestry. All vertebrate embryos, for example, have gill slits and tails. All of the embryos in Figure (PageIndex4), except for fish, lose their gill slits by adulthood, and some of them also lose their tail. In humans, the tail is reduced to the tail bone. Thus, similarities organisms share as embryos may no longer be present by adulthood. This is why it is valuable to compare organisms in the embryonic stage.

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Figure (PageIndex4): Embryos of different vertebrates look much more similar than the animals do at later stages of life. Rows I, II, and III illustrate the development of the embryos of fish on the far left, salamander, tortoise, chick, hog, calf, rabbit, and human on the far right, from the earliest to the latest stages.

Vestigial Structures

Structures like the human tail bone are called vestigial structures. Evolution has reduced their size because the structures are no longer used. The human appendix is another example of a vestigial structure. It is a tiny remnant of a once-larger organ. In a distant ancestor, it was needed to digest food, but it serves no purpose in the human body today. Why do you think structures that are no longer used shrink in size? Why might a full-sized, unused structure reduce an organism’s fitness?



Comparing DNA

Darwin could compare only the anatomy and embryos of living things. Today, scientists can compare their DNA. Similar DNA sequences are the strongest evidence for evolution from a common ancestor. Look at the diagram in Figure (PageIndex5). The diagram is a cladogram, a branching diagram showing related organisms. Each branch represents the emergence of new traits that separate one group of organisms from the rest. The cladogram in the figure shows how humans and apes are related based on their DNA sequences.

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Figure (PageIndex1): Figure (PageIndex5): Cladogram of Humans and Apes. This cladogram is based on DNA comparisons. It shows how humans are related to apes by descent from common ancestors. Humans are most closely related to chimpanzees and Bonobo (our common ancestor existed most recently). We are less closely related to gorillas, and even less closely related to Orangutan.

invernessgangshow.netgeography of Camels: An Example

Today, the camel family includes different types of camels (Figure (PageIndex6)). All of today’s camels are descended from the same camel ancestors. These ancestors lived in North America about a million years ago.

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Early North American camels migrated to other places. Some went to East Asia via a land bridge during the last ice age. A few of them made it all the way to Africa. Others went to South America by crossing the Isthmus of Panama. Once camels reached these different places, they evolved independently. They evolved adaptations that suited them for the particular environment where they lived. Through natural selection, descendants of the original camel ancestors evolved the diversity they have today.

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Figure (PageIndex6). Camel Migrations and Present-Day Variation. Members of the camel family now live in different parts of the world. Dromedary camels are found in Africa, Bactrian camels in Asia, and Llamas in South America. They differ from one another in a number of traits. However, they share basic similarities. This is because they all evolved from a common ancestor. What differences and similarities do you see?