7.7 - Common Ancestry

Evidence for Common Ancestry of All Eukaryotes

1. Membrane-Bound Organelles:
• Observation: All eukaryotic cells have a nucleus and other membrane-bound organelles, like the endoplasmic reticulum, Golgi apparatus, and mitochondria. Prokaryotes don't have membrane-bound organelles.
• Significance: Nearly all eukaryotes have mitochondria (or reduced versions of them), and the evidence points to mitochondria being gained once, by endosymbiosis, in the ancestor of all eukaryotes (see Topic 2.11).

2. Linear Chromosomes:
• Observation: Eukaryotes store their DNA in linear chromosomes, with the DNA wrapped around histone proteins and telomeres at the ends. Most prokaryotes have a single circular chromosome.
• Significance: Every eukaryotic group packages its chromosomes in the same basic way, which points to a shared origin.

3. Genes That Contain Introns:
• Observation: Eukaryotic genes contain introns, noncoding sections that are cut out of pre-mRNA by spliceosomes before translation (see Topic 6.3). Prokaryotic genes don't have introns that are removed this way.
• Significance: Splicing is a complicated process that uses the same kind of machinery in all eukaryotes. It's very unlikely to have evolved separately in each group, so it was most likely inherited from a common ancestor.

• Some features are shared by every living thing, prokaryotes and eukaryotes alike: DNA as the genetic material, nearly the same genetic code, ribosomes that make proteins, and ATP as the energy source for cell work. These point to an even older common ancestor of all life.