Explain how multiple lines of evidence (fossil, anatomical, and molecular) support common ancestry and biological evolution (North Carolina Standard Course of Study, Biology, LS.Bio.9).

What this topic is asking

North Carolina LS.Bio.9 asks how multiple lines of evidence support common ancestry and evolution. For the Biology EOC you need to know the fossil record, homologous and vestigial structures, embryology, and molecular (DNA and protein) evidence, and to understand that the strength comes from many independent lines agreeing. Items often ask you to classify a type of structure or interpret shared DNA.

The fossil record

Fossils in deeper (older) rock layers tend to be simpler and more different from living organisms, while those in shallower (younger) layers are more like modern forms. This shows that life has changed over time, the basic claim of evolution. Especially powerful are transitional fossils, which show features intermediate between two groups (for example, fossils linking fish and early land animals, or dinosaurs and birds), filling in the path of change.

Anatomical evidence: homologous and vestigial structures

The homologous-versus-analogous distinction is a common EOC item. The test is whether the structure is shared (homologous, common ancestry) or only the function is shared (analogous, no common origin).

Embryological and molecular evidence

Two further lines complete the picture, and the molecular evidence is the most decisive.

• Embryology. Related species often look very similar as early embryos, sharing features (such as gill-like structures and tails) that point to common ancestry, even when the adults look different.
• Molecular evidence. All life uses DNA and the same genetic code, which itself suggests common ancestry. More specifically, the more similar the DNA and protein sequences of two species, the more closely related they are. Comparing sequences lets scientists measure relatedness precisely and build evolutionary trees, which is why molecular evidence is considered the strongest line.

The real force of the argument is that these independent lines, fossils, anatomy, embryos, and molecules, all point to the same relationships. Independent methods agreeing is what makes the evidence convincing.

Try this

Q1. Explain what homologous structures suggest about the species that have them. [2]

• Cue. They share the same underlying structure because of a common ancestor, so the species are evolutionarily related.

Q2. State why molecular evidence is considered strong evidence for evolution. [2]

• Cue. The degree of similarity in DNA and protein sequences directly measures relatedness, and all life sharing DNA and the genetic code points to common ancestry.