Lecture 18- Life on Earth Nov. 17, 2016
Most recent common ancestor (MRCA) for a given group(s), given a phylogenetic tree.
- Common ancestry was the main point in Darwin’s Origins. All cellular life has the same genetic
code and can be traced back to find that there is a point at which we all shared a common
ancestor (bacteria, archaea, eukarya). This is called LUCA- last universal common ancestor for a
group of organisms
- How long ago was it that the MRCA existed for those groups? With any combination of species
on Earth. This shows the degree of relatedness (eg. humans and bacteria are not closely related
as you have to go back far for the MRCA).
- Branching points on the tree called “rendezvous” points.
Why the idea that “humans are descended from chimps” is inaccurate.
- If we are descended from chimps, why do they still exist on Earth? Our most recent common
ancestor was not human but not chimp either.
Why some traditional groupings of organisms (“reptiles”, “fish”) do not reflect
- The Old world monkeys are more closely related to us than to New World monkeys! Therefore,
the group “monkeys” does not represent evolutionary relationships.
- ~18000 new species of life discovered each year. Discoveries can be of animals we have never
seen before, those we have seen but just recently genetically identified as different from their
relative species, or bones of species who have gone extinct.
- 1.1-1.2 million species we have named…. About ~8.7 million eukaryotes predicted to exist on
Earth. We have only named/ described up to 10%.
- Doing the math, every generation should double your relatives. So 30 generations back will have
2^30 relatives. But that many people didn’t exist back in that generation?!
- If we go back far enough, any two individuals will have a common ancestor.
Rendezvous Zero: Humans Themselves
- The MCRA of all humans Lecture 18- Life on Earth Nov. 17, 2016
- How long ago was that? Using mathematical modelling, estimated to be about 3000 years ago/
Rendezvous One: Chimps and Bonobos
- Our evolutionary cousins
- Closest relatives
- MCRA 6 million years