AP Biology · Unit 7 · Topic 7.7
Some features are shared by every living thing and some only by eukaryotes. Which is which is the whole of this topic.
Learning objective 7.7.A Describe structural and functional evidence on cellular and molecular levels that provides evidence for the common ancestry of all eukaryotes.
This topic asks a narrower question than it looks. Not “what shows that life shares an ancestor” but “what shows that all eukaryotes do”. Those are different questions with different answers, and the difference is visible in this grid: a feature is evidence about whichever group has it and only that group.
| Evidence | What it supports | What it does not support |
|---|
The genetic code is a lookup table with 64 entries, and essentially every organism on Earth uses the same one. Below is the start of the human beta-globin gene. Step through it and watch a table that a bacterium uses too turn human DNA into a human protein.
There is nothing chemically necessary about the assignments. Nothing about the codon GTG makes it mean valine; a code that read GTG as tryptophan would work exactly as well, provided the tRNAs matched. The number of possible assignments of twenty amino acids and a stop signal across 64 codons is a number with more than eighty digits.
Two lineages that originated separately would have had to land on the same one. Inheriting it from a shared ancestor requires no coincidence at all, which is why the shared code is treated as among the strongest single pieces of evidence in biology.
A mitochondrion sits inside a eukaryotic cell. On each trait below, ask which it resembles more: the cell it lives in, or a free-living bacterium.
| Trait | Free-living bacterium | Mitochondrion | Eukaryotic nucleus |
|---|
The mitochondrion's bacterial features are evidence about the mitochondrion's ancestry, not about the whole eukaryotic cell's. The cell that swallowed it already had a nucleus, linear chromosomes and introns; it was already a eukaryote. Endosymbiosis explains where the organelle came from, not where eukaryotes came from.
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