Evidence for Evolution

1. Evidence for Evolution

2. Signs of evolution • Evolution leaves observable signs. Such clues to the past are essential to any historical science. Biological evolution has left marks on all aspects of life—in the fossil record and in the diverse assortment of modern species.

3. The Fossil Record • Preserved remains or markings left by organisms that lived in the past are called fossils. • Most fossils are found in sedimentary rocks. Sand and silt eroded from the land are carried by rivers to seas and swamps, where the particles settle to the bottom. • Over millions of years, deposits pile up and compress the older sediments below into rock. Rock strata, or layers, form when the rates of sedimentation or the types of particles forming the sediments vary over time.

4. The Fossil Record • Aquatic organisms can become fossils when they die and are buried in sediments in a way that preserves some of their structure. • Land organisms can become fossils in a similar way, if they are swept into rivers, lakes, swamps, and seas. • Other remains of land dwellers may become fossils after being covered by windblown dust, sand, or volcanic ash.

5. The Fossil Record • The fossil record provides evidence of Earth's changing life. • The oldest fossil evidence of life consists of chemical traces in rocks from Greenland that are 3.8 billion years old. • Fossils of prokaryotes (bacteria and archaea) have been found in rocks of about 3.5 billion years in age. These data fit with the molecular and cellular evidence that prokaryotes are the oldest form of life. • Fossils in younger layers of rock record the evolution of various groups of eukaryotic organisms. Fossils of species that became extinct scientists reconstruct the past

6. Fossil Record • Paleontologists have discovered fossils of many ancestral life forms that link past and present. • For example, fossil evidence supports the hypothesis that whales, which have no hind limbs, evolved from land-dwelling ancestors that had four limbs. • Paleontologists digging in Egypt and Pakistan have identified ancient whales that had hind limb bones. • These whales, which lived about 40 million years ago, were aquatic animals that no longer used their legs to support their weight. Even larger leg bones are found in fossils of older whale species that may have split their time between living on land and in water.

7. Fossil Records

8. Geographic Distribution • The differences and similarities between organisms in different parts of the world were some of the first observations that Darwin made on his voyage. These observations suggested to Darwin that today's organisms evolved from ancestral forms.

9. Similarities in Structure • Certain similarities in structure among species provide clues to evolutionary history. • For example, the forelimbs of all mammals consist of the same skeletal parts. Human arms, cat forelegs, whale flippers, and bat wings all have the same basic combination of bones • The functions of these forelimbs differ, however. You know that a whale's flipper does not do the same job as a bat's wing.

10. Similarities in Structure • Since the functions of these limbs are completely different, you might expect that their structures would also be entirely different. Yet, that is not the case. Arms, forelegs, flippers, and wings of different mammals are variations on a common structural theme—one that has become adapted to different functions. Such similar structures in species sharing a common ancestor are called homologous structures.

11. Similarities in Structure

12. Similarities in Structure • Homologous structures support other evidence that evolution is a remodeling process. • Structures that originally functioned one way in ancestral species become modified as they take on new functions. • This idea is what Darwin meant by "descent with modification."

13. Similarities in Structure • Some of the most interesting homologous structures are those that have a major function in one species but are not important in a related species. • Vestigial structures are remnants of structures that may have had important functions in an ancestral species, but have no clear function in some of the modern descendants. Often, vestigial organs are reduced in size. • For example, the whales of today lack hind limbs, but some have small vestigial hipbones probably derived from their four-footed ancestors described earlier..

14. Similarities of Structure • Lamarck's idea of inheritance of acquired characteristics could account for the reduced size of vestigial structures. • However, genetic evidence does not support such a process of inheritance. • Natural selection provides a different explanation for vestigial structures that is consistent with known processes of inheritance. Natural selection would favor the survival and reproduction of individuals with genes for reduced versions of those structures. Consider also that if species arose independently, remnants of structures similar to working organs in other species would be unlikely. But the presence of these structures makes sense if certain species descended from a common ancestor

15. Molecular Biology • In recent decades, biologists have been reading a molecular history of evolution in the DNA sequences of organisms. • The sequences of bases in DNA molecules are passed from parents to offspring. • These DNA sequences determine the amino acid sequences of proteins. These information-rich molecules are the records of an organism's ancestry (hereditary background). • Among siblings, the DNA and protein sequences are very similar. • However, the sequences of unrelated individuals of the same species show more differences.

16. Molecular Biology • This idea of molecular comparison extends to studying relationships between species. • If two species have genes and proteins with sequences that match closely, biologists conclude that the sequences must have been inherited from a relatively recent common ancestor. • In contrast, the greater the number of differences in DNA and protein sequences between species, the less likely they share as close a common ancestry.

17. Molecular Biology • Testable hypotheses are at the heart of science. DNA and protein analyses are new tools for testing hypotheses about evolution. • For example, fossil evidence and studies of anatomy support the hypothesis that humans, chimpanzees, and gorillas are closely related. This hypothesis is testable. It can be used to make predictions about what to expect if the hypothesis is correct.

18. Molecular Biology • For example, if humans and other primates are closely related, then they should share much of their inherited DNA and proteinsequences. The sequences of distantly related species should have more differences. If molecular analysis did not confirm this prediction, that would cast doubt on the hypothesis that apes and humans are closely related.

19. Molecular Biology • The following figure compares the amino acid sequence of human hemoglobin (the protein that carries oxygen in blood) with the hemoglobin of other vertebrates.

20. Molecular Biology

21. Molecular Biology • The data support the hypothesis that humans are more closely related to primates than to other vertebrates. Other evidence comes from DNA sequences in humans and chimpanzees. There is approximately 5 percent difference in the total DNA between these two species.

22. Molecular Biology • Darwin's boldest hypothesis was that all life forms are related. • The molecular evidence includes the common genetic code shared by all species. This genetic language has been passed along through all the branches of evolution. And, it has added to the evidence that supports evolution as an explanation for the unity and diversity of life.