Analyze and interpret evidence from fossils, anatomy, embryology, and molecular biology that supports common ancestry (Tennessee Academic Standards for Science, Biology I, BIO1.LS4).

What this topic is asking

The Tennessee LS4 standards (Biological Change) ask you to analyze and interpret evidence that living things share common ancestors. For the Biology I EOC that means knowing the main lines of evidence, the fossil record, homologous and vestigial structures, embryological similarities, and molecular (DNA and protein) evidence, and being able to read data (such as a table of DNA differences) and explain what it shows. Items frequently give a comparison and ask which species are most closely related.

The fossil record

The fossil record shows that life on Earth has changed over time: deeper (older) rock layers contain simpler or different organisms than shallower (younger) layers. It also preserves transitional forms, fossils with features of two groups (such as fossils linking fish to early land animals, or dinosaurs to birds), which show change between forms. The EOC uses the fossil record to teach that species are not fixed but have a history.

Homologous, analogous, and vestigial structures

Comparing anatomy across species gives three key terms the EOC tests:

• Homologous structures share the same underlying structure because of common ancestry, even if they now do different jobs. The classic example is the forelimb of a human (arm), whale (flipper), bat (wing), and cat (leg): all have the same arrangement of bones, inherited from a shared ancestor.
• Analogous structures share a function but not structure or ancestry, because they evolved separately for the same purpose. A bird's wing and an insect's wing both enable flight but are built differently and are not evidence of close ancestry.
• Vestigial structures are reduced remnants of features that were functional in an ancestor (such as the human tailbone or the hip bones of a whale). Their presence is evidence of descent from ancestors that used those features.

Embryological similarities

Closely related species often look strikingly similar as early embryos, sharing features (such as a tail and pharyngeal pouches in vertebrate embryos) that may not appear in the adult. These similarities during development are evidence that the species share a common ancestor whose developmental pattern they inherited.

Molecular evidence: the strongest line

The most powerful modern evidence comes from comparing DNA and protein sequences. Because all life uses the same genetic code and similar molecules, the degree of similarity in DNA or proteins reflects how recently two species shared a common ancestor:

• The more similar the sequences, the more recently the species diverged from a common ancestor.
• The more differences that have accumulated, the longer ago they shared an ancestor.

So a table of DNA differences lets you rank relatedness: the species with the fewest differences from a given organism is its closest relative. This molecular evidence agrees with the anatomical and fossil evidence, which is why common ancestry is so strongly supported.

Try this

Q1. Explain the difference between homologous and analogous structures, with an example of each. [3]

• Cue. Homologous structures share the same underlying structure from a common ancestor (the forelimb bones of a human and a whale); analogous structures share a function but not structure or ancestry (a bird's wing and an insect's wing).

Q2. State what the fossil record shows about life on Earth. [2]

• Cue. That life has changed over time, with older rock layers containing different (often simpler) organisms than younger layers, and that transitional forms link groups.