Animal Diversity

Students apply their knowledge of life and heredity to the theory of evolution, exploring how the fossil record, DNA analysis, and embryonic comparisons provide evidence of common ancestry and diversity.

• Evolution is the theory that describes the changes in the heritable traits of a population of organisms as successive generations replace one another.
• The fossil record provides evidence of evolution because it shows that there are anatomical similarities between ancient organisms and modern-day organisms.
• Many of the fossils that scientists have found tell of ongoing changes to life over time, becoming more complex as life evolved.
• There are limitations in what fossil evidence can tell scientists about evolutionary relationships.
• Embryos are the early stages of many developing multicellular organisms, and scientists have known for a long time that species that looked quite different as adults often had many similarities when they were embryos.

• What kinds of information can a fossil provide?
• How can scientists use rock layers to help them find the relative age of a fossil?
• Why do some fossils tell scientists of organisms that are now extinct?
• Why do scientists look for evidence of similar traits rather than different traits to understand relationships between different organisms?
• Why can’t fossils tell scientists definitively about past changes on Earth?
• How can DNA also help scientists determine evolutionary relationships?
• What does it mean to share 60 percent of our DNA with fruit flies, and 99 percent with chimpanzees?
• What are some patterns of similarities in early-stage embryos?
• How do these similarities support the argument that animals are all related to one another?

Misconception: Dominant traits are the most common traits in a population.

Fact: Whether a gene is dominant or recessive has no bearing on how common it is in a population. Instead, a dominant trait will be expressed over a recessive trait.

Embryo : the early stages of many developing multicellular organisms

Evolution : changes in the heritable traits of a population of organisms as successive generations replace one another; populations of organisms evolve, not individual organisms

Extinction : when there are no longer any living members of a species in existence

Population : all the members of a species within a particular area

Searching for Ancient Fish Fossils

Neil Shubin has spent almost two decades searching the Arctic for ancient fish bones. He is looking for evidence for life’s transition from the ocean to the land.

Most scientists believe that for the first several billion years that life existed on Earth, all organisms lived in the ocean. Then, the fossil record tells us that around 365 million years ago, many land animals appeared. But scientists didn’t know exactly how this change came about. How did animals living in the deep ocean end up with traits that allowed them to transition onto the land—and survive?

A Surprising Discovery

Shubin spent years examining ancient rocks. He was searching for clues in fossils that might help to answer this question. Then, in 2004, Shubin and a team of scientists made a surprising discovery. They were studying 375- million-year-old rocks in the Canadian Arctic. They saw a fossilized snout of a fish sticking out of the rocks. This fish head was unlike other fish heads because it was flat. This was the first clue for the scientists that they might be onto something unusual.

Shubin and his team carved out the rock holding the fish fossil and sent it back to their lab. They wanted to study it. When the scientists announced their discovery in 2006, it made news headlines around the world. The fossil looks like a cross between an ancient fish and the first four-legged animals that appeared on Earth. For example, it has fins, scales, and gills. Fish have these traits but land animals don’t. However, it also has a neck and a flat head, which fish don’t have but the earliest land animals did.

How Life Evolves

The fossil discovery by Shubin’s team has provided scientists with evidence for how millions of years ago, ocean-dwelling creatures evolved into animals that could survive on land. Evolution is the theory that describes the changes in the heritable traits of a population of organisms as successive generations replace one another. Remember that traits are physical or behavioral characteristics of an organism.

Broadly speaking, the theory of evolution says that all life on Earth descended from a common ancestor. As the earliest life forms reproduced and passed along their genetic information stored in DNA to their offspring, genetic variation appeared. This variation was first caused by mutations (permanent changes to an individual’s DNA). It was later caused by sexual reproduction and the exchange of genetic information between different populations. A population is all the members of a species within a particular area.

This genetic variation meant that some individuals had traits that were better suited to their environment than others. Those individuals were more likely to survive and pass along their traits to offspring. We’ll explore this concept in more detail in the next section.

Over the billions of years since life first appeared on Earth, scientists believe that evolution has caused all of the diversity of life forms around us today. Over many generations, different kinds of organisms evolved, each with a unique set of traits that help it survive in its environment.

Studying Arctic Rocks

Shubin’s team focused on a region in the Canadian Arctic that had rocks roughly 375 million years old. They focused on this region because they wanted to find evidence of the first animals that had transitioned from the water to the land. They already had fossil evidence showing that by 365 million years ago, there were many different kinds of land animals including ancient amphibians and reptiles. So they were looking for fossils from a time period before that.

They also wanted to find rocks that were formed in bodies of water such as oceans, lakes, or streams. This is because they knew that an animal that transitioned from the water to the land would have spent at least some time in a body of water. This would make these rocks more likely to hold fossils of this animal.

Constructing Life’s History

Scientists can use these different pieces of evidence to help piece together the story of life’s evolution. Each piece of evidence tells a different part of the story. Together, scientists can construct a timeline for how life has evolved over time.

For example, fossils can tell scientists about similarities and differences in the structures of ancient organisms. They can also tell of past environmental changes. This includes times in Earth’s history when oceans covered much of the land, and then the oceans dried up, leaving behind rocks and fossils to tell of those changes. Some of the evidence tells us that only a very small number are still alive today. Many organisms that once lived are now extinct. Extinction occurs when there are no longer any living members of a species in existence.

Whales tell an interesting story of evolution. Whales live in the ocean, but they aren’t fish. Whales are mammals, just like people. Being a mammal means they breathe air, they are warm-blooded, they give birth to live young, and they nurse their babies with milk.

Scientists believe that around 55 million years ago, there was so much food in the water that a group of land mammals began spending more time in the water. Over millions of years, this group evolved into modern whales, which live entirely in the ocean. Scientists use various pieces of evidence to support this theory of how the whale evolved. Evidence includes the fact that whale embryos have hind legs and the DNA of whales is very similar to the DNA of other mammals. Scientists have also found multiple fossils that show how the structures of the animals have evolved over time to become ocean-dwelling mammals.

For the main hands-on activity in this lesson, students design investigations where they analyze similarities in the anatomies of modern and fossil organisms, embryo development, and DNA patterns among various species to figure out evolutionary relationships. Students conduct these investigations to figure out how different species are related to one another because of evolution.