evidence for evolution

vestigial structures

Some of the most interesting evidence for evolution comes from vestigial structures. These are body parts that are no longer used but are still present in modern organisms. Examples in humans include tail bones and the appendix. Human beings obviously dont have tails, but our ancestors did. We still have bones at the base of our spine that form a tail in other, related animals, such as monkeys. The appendix is a tiny remnant of a once-larger organ. In a distant ancestor, it was needed to digest food. If your appendix becomes infected, a surgeon can remove it. You wont miss it because it no longer has any purpose in the human body.

comparing body parts

Comparing body parts of different species may reveal evidence for evolution. For example, all mammals have front limbs that look quite different and are used for different purposes. Bats use their front limbs to fly, whales use them to swim, and cats use them to run and climb. However, the front limbs of all three animalsas well as humanshave the same basic underlying bone structure. You can see this in Figure 7.11. The similar bones provide evidence that all four animals evolved from a common ancestor.

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what are fossils

Fossils are the preserved remains or traces of organisms that lived during earlier ages. Remains that become fossils are generally the hard parts of organismsmainly bones, teeth, or shells. Traces include any evidence of life, such as footprints like the dinosaur footprint in Figure 7.7. Fossils are like a window into the past. They provide direct evidence of what life was like long ago. A scientist who studies fossils to learn about the evolution of living things is called a paleontologist.

how fossils are dated

Fossils are useful for reconstructing the past only if they can be dated. Scientists need to determine when the organisms lived who left behind the fossils. Fossils can be dated in two different ways: absolute dating and relative dating. Absolute dating determines about how long ago a fossil organism lived. This gives the fossil an approximate age in years. Absolute dating is often based on the amount of carbon-14 or other radioactive element that remains in a fossil. You can learn how carbon-14 dating works by watching this short video: Relative dating determines which of two fossils is older or younger than the other but not their age in years. Relative dating is based on the positions of fossils in rock layers. Lower rock layers were laid down earlier, so they are assumed to contain older fossils. This is illustrated in Figure 7.9.

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how fossils form

The soft parts of organisms almost always decompose quickly after death. Thats why most fossils consist of hard parts such as bones. Its rare even for hard parts to remain intact long enough to become fossils. Fossils form when water seeps through the remains and deposits minerals in them. The remains literally turn to stone. Remains are more likely to form fossils if they are covered quickly by sediments. Once in a while, remains are preserved almost unchanged. For example, they may be frozen in glaciers. Or they may be trapped in tree resin that hardens to form amber. Thats what happened to the wasp in Figure 7.8. The wasp lived about 20 million years ago, but even its fragile wings have been preserved by the amber.

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evidence from living organisms

Scientists have learned a lot about evolution by comparing living organisms. They have compared body parts, embryos, and molecules such as DNA and proteins.

using fossils to understand evolution

The evolution of whales is a good example of how fossils can help us understand evolution. Scientists have long known that mammals first evolved on land about 200 million years ago. Its been a mystery, however, how whales evolved. Whales are mammals that live completely in the water. Did they evolve from earlier land mammals? Or did they evolve from animals that already lived in the water? Starting in the late 1970s, a growing number of fossils have allowed scientists to piece together the story of whale evolution. The fossils represent ancient, whale-like animals. They show that an ancient land mammal made its way back to the sea more than 50 million years ago. It became the ancestor of modern whales. In doing so, it lost its legs and became adapted to life in the water. In Figure 7.10 you can see an artists rendition of such a whale ancestor. It had legs and could walk on land, but it was also a good swimmer. Watch this short video to learn more about the amazing story of whale evolution based on the fossils:

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comparing molecules

Scientists can compare the DNA or proteins of different species. If the molecules are similar, this shows that the species are related. The more similar the molecules are, the closer the relationship is likely to be. When molecules are used in this way, they are called molecular clocks. This method assumes that random mutations occur at a constant rate for a given protein or segment of DNA. Over time, the mutations add up. The longer the amount of time since species diverged, the more differences there will be in their DNA or proteins. Table 7.1 compares the DNA of four different organisms with modern human DNA. The DNA of chimpanzees is almost 99 percent the same as the DNA of modern humans. This shows that chimpanzees are very closely related to us. We are less closely related to the other organisms in the table. Its no surprise that grapes, which are plants, are less like us than the animals in the table. Organism Chimpanzee Cow Chicken Honeybee Grape Similarity with Human DNA (percent the same) 98.8 85 65 44 24

comparing embryos

An embryo is an organism in the earliest stages of development. Embryos of different species may look quite similar, even when the adult forms look very different. Look at the drawings of embryos in Figure 7.12. They represent very early life stages of a chicken, turtle, pig, and human being. The embryos look so similar that its hard to tell them apart. Such similarities provide evidence that all four types of animals are related. They help document that evolution has occurred.

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observing evolution in action

The best evidence for evolution comes from actually observing changes in organisms through time. In the 1970s, biologists Peter and Rosemary Grant went to the Galpagos Islands to do fieldwork. They wanted to re-study Darwins finches. They spent the next 40 years on the project. Their hard work paid off. They were able to document evolution by natural selection taking place in the finches. A period of very low rainfall occurred while the Grants were on the islands. The drought resulted in fewer seeds for the finches to eat. Birds with smaller beaks could eat only the smaller seeds. Birds with bigger beaks were better off. They could eat seeds of all sizes. Therefore, there was more food available to them. Many of the small-beaked birds died in the drought. More of the big-beaked birds survived and reproduced. Within just a couple of years, the average beak size in the finches increased. This was clearly evolution by natural selection.

instructional diagrams

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questions

Remains of organisms become fossils when their organic matter is replaced by

a. traces

b. bones

c. amber

-->  d. minerals

_Evidence for evolution includes millions of fossils.

-->  a. true

b. false

_Fossils generally form from the hard parts of organisms.

-->  a. true

b. false

The relative ages of fossils are based on their

a. similarities in DNA

b. amounts of carbon-14

-->  c. positions in rock layers

d. approximate age in years

Which statement about the evolution of mammals is true?

a. Mammals first evolved in the ocean

b. The earliest mammals evolved about 2 million years ago

-->  c. All modern mammals have the same bones in their front limbs

d. all of the above

_It is very common for dead organisms to become fossils.

a. true

-->  b. false

_Remains are less likely to become fossils if they are covered quickly by sediments.

a. true

-->  b. false

Which organism has DNA that is least like human DNA?

a. honeybee

b. chicken

-->  c. grape

d. cow

_Some fossils form when dead organisms are frozen in glaciers.

-->  a. true

b. false

The best evidence for evolution is the

a. fossil record

b. comparison of embryos

c. data from radioactive elements

-->  d. observation of traits changing through time

_Relative dating can be used to determine how long ago a fossil organism lived.

a. true

-->  b. false

_Fossil evidence shows that whales evolved from mammals that had always lived in the ocean.

a. true

-->  b. false

The footprints of animals are sometimes preserved as fossils.

-->  a. true

b. false

Carbon-14 dating is a method of relative dating.

a. true

-->  b. false

Some fossils form when organisms are preserved in amber.

-->  a. true

b. false

Whales evolved from a four-legged ancestor.

-->  a. true

b. false

Closely related animals generally look more similar as they grow older.

a. true

-->  b. false

___any method of estimating the age of fossils that determines only which of two fossils is older or

a. amber

b. molecular clock

-->  c. relative dating

d. embryo

e. trace

f. absolute dating

g. vestigial structure

___any evidence other than organic remains that a living organism leaves behind

a. amber

b. molecular clock

c. relative dating

d. embryo

-->  e. trace

f. absolute dating

g. vestigial structure

___molecule that is compared among species to estimate how long it has been since they diverged from

a. amber

-->  b. molecular clock

c. relative dating

d. embryo

e. trace

f. absolute dating

g. vestigial structure

___hardened tree resin

-->  a. amber

b. molecular clock

c. relative dating

d. embryo

e. trace

f. absolute dating

g. vestigial structure

___body part that is no longer used but is still present in modern organisms

a. amber

b. molecular clock

c. relative dating

d. embryo

e. trace

f. absolute dating

-->  g. vestigial structure

___any method of estimating the age of fossils that provides an approximate age in years

a. amber

b. molecular clock

c. relative dating

d. embryo

e. trace

-->  f. absolute dating

g. vestigial structure

___very early stage of development of an organism

a. amber

b. molecular clock

c. relative dating

-->  d. embryo

e. trace

f. absolute dating

g. vestigial structure

Most of what we know about dinosaurs is based on

a. molecular data

b. similarities in embryos

c. vestigial organs

-->  d. fossils

Which of the following parts of animals are most likely to be preserved as fossils?

a. skin

b. feathers

c. hair

-->  d. teeth

The front limbs of whales, bats, and cats

a. look very different

b. are used for different purposes

c. have the same basic bone structure

-->  d. all of the above

The use of molecular clocks assumes that

-->  a. more similar molecules reflect closer relationships

b. mutations occur at an increasing rate for a given molecule

c. most molecules are identical in all living species

d. all of the above

The Grants observed an increase in the average size of finch beaks during a

-->  a. drought

b. hurricane

c. cold spell

d. volcanic eruption

Fossils most often form when minerals in water turn the remains of organisms to

a. vestigial structures

b. sediments

c. bones

-->  d. stones

What percent of chimpanzee DNA is the same as human DNA?

a. 24.4

b. 44.4

c. 88.8

-->  d. 98.8

diagram questions

No diagram questions associated with this lesson