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

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.

• can you give us some sense of what you've tried? what is the phylogeny you're using, if any? what's the level of resolution you want? For example, if you look e.g. at this little tree (researchgate.net/profile/Aiasha-Siddika-Arshi/publication/…), you can find an informatively labeled node, suggesting that the answer might be "the coelomate common ancestor". Another way would be to find an animal phylogeny, read it into R with read.tree, and then use mrca(). IDK which way you want. Jan 1 at 21:50
• Sorry I’m a total novice at this I’ll digest your response and get back to you later. Jan 1 at 21:55
• I guess, it would help to know exactly what you're trying to do. If you're looking for a single piece of factual information, there is a knowable right answer here that you can just look up (which I believe is "the coelomate common ancestor"). However, the fashion that you phrase this and the forum that you post it suggest that you want a more general workflow. Jan 1 at 23:04
• for example, see this web app which computes common ancestors: timetree.org. To state it a different way, your specific lobster/human question is finding the common ancestor of deuterostomes and protostomes in figure 1 here: onlinelibrary.wiley.com/doi/epdf/10.1111/…. I recommend that last paper if you want a general sense of how this stuff works. Jan 1 at 23:21

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:

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

2. 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)
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.

• This is really helpful. I’ll have a good look at this tomorrow and give you a little more detail on what I’m trying to do. Jan 2 at 0:02