What is the most appropriate way to find the most recent common ancestor between two distantly related species

Question

I want to specifically find the common ancestor between a lobster and a humans. I suspect it was an aquatic worm of some description. But I want to know about the nervous system of this common ancestor.

Answer 1

Because I've now posted several comments, I'll just roll them all up.

For background on the approaches used to identify most recent common ancestors and a high-level look at how animal taxonomy has been inferred, I suggest Lynch 1999.

I think that there are 2 interpretations of this question. If you are interested in just looking up a single MRCA of well-defined clades, such as lobster and human, here are some approaches:

Easy way:

1. Look at a tree diagram, e.g. this:

2. Find the tips that correspond to your species of interest (arthropods for lobster, chordata for humans).

3. Find where they join together in the diagram (the branch labeled "true coelom").

You have your answer, the MRCA is the group of organisms with a true coelom, coelomates.

A more involved way using a database

1. Go to this website.
2. Find the group of species 1 (arthropods, protostomes, etc. for lobster, chordata, deuterostomes etc. for human)
3. navigate around until you see the group containing the two groups (in this case listed as "bilateria"). In this case you are looking for the bilaterian common ancestor.

another database

1. Go to this website.
2. Point and click your way to a view where you see your 2 clades of interest (arthropods, chordates in this case). See figure.
3. Find where they join (in this case, it is less certain about the existence of a coelomate common ancestor, so it just says "bilaterians").

In R

We might also want a programmatic procedure in case we need to automate it. The following gives a series of commands in R that do this.

Get a newick/nexus tree from somewhere e.g. here of animals (go "control panel --> export, format = "newick tree" --> download).

For this tree, we can approximate an answer as following:

library(ape)
it = read.tree("~/Downloads/zUtVMYn3wJQBoP7E8fygTw_newick.txt")  # autogenerated filename from the website.
getMRCA(it, c("Homo_sapiens", "Drosophila_melanogaster"))  # drosophila is one of only 2 arthropods in this tree, but arthropods all have an arthropod common ancestor
# prints "200", the node index of the MRCA.

You can use that node index to do things like look up a name in the node.label attribute of the tree (it) structure, or to do other operations on the tree.

Answer 2

Good answer.

The common ancestor is not known due to the paucity of the fossil record at this point in evolutionary time. A reconstruction can be made using coalescent theory which is what @MaximillanPress describes. The level of uncertainty at that depth of calculation requires significant expertise. I would re-evaluate the comparison with related organisms that are heavily studied.

Lobsters are decapods (Decapoda) and closely related to insects, moreover they are monophyletic with the Order Insecta. A lot is known about a lot of insects, many being model organisms, such as Drosophila melanogaster. Both decapods and insects exist within the arthropods (Arthropoda), along with spiders and the like. Their ancestors were marine arthropods, trilobites are arthropods. The major diversity of the arthropods was generated in the Cambrian Explosion and a lot of work has gone into identifying these fossils.

Traversing between decapods and insects is as far as I'd go, which is likely the late stages of the Cambrian explosion. There are likely to be putative ancestral fossils - not that you can get any molecular sequence from them however. The closest living ancestor on the insect side is Colembola (beach hoppers).

There are serious difficulties in major molecular calculations between decapods and mammals.