Where did I come from?


Phylogenetic Relationships among Humans

Allan Wilson and his colleagues Mark Stoneking and Rebecca Cann estimated the phylogenetic relationships among the humans whom they sampled. Here you will need to examine their results closely to decide which of the competing hypotheses is supported. There are two parts: examining the phylogenetic relationships among modern human populations and estimating the age of the common ancestor of all modern humans, and of all non-Africans.

Phylogenetic Relationships
One of the major outcomes of this study is a phylogenetic tree of relationships. You can download this tree, and examine it closely to decide which hypothesis it supports.

Get Figtree
Take a few minutes now to download and install FigTree, software for viewing and manipulating phylogenetic trees. Return to this window when you are ready to proceed. [Get FigTree]

Examine and Explore the Tree
Begin your exploration of the tree by downloading it.   Cann87tree (1 KB) choose Save the Link As ... and save the file on your desktop computer.  (Please extract zip file before use)
Now, start the FigTree software. Once it is started, from the File menu choose Open and select the tree file which you just downloaded.

The individual samples are coded with the geographic region of origin (Af = Africa, As = Asia, Au = Australia, Eu = Europe, NG = New Guinea).
The tree looks pretty complicated! [Have a peek.] How are you going to interpret this and compare it with the predicted trees?

There are some things that you can do to make the tree more manageable:

  • Zoom in and Expand - make the details of the tree easier to read
    FigTreeZoom.png
  • Order nodes - arrange the tree by rotating clades (branches) to give it a tidy appearance
    FigTreeOrderNodes.png  
  • Colour the samples - select all of the samples from each region and give them a distinctive colour 
    FigTreeColourTaxa.png

These actions should be bringing your tree into a more readable form, perhaps something like [this].

Interpret the Tree
Now you need to assess the tree, to decide whether it supports one or the other of the two hypotheses of human ancestry. Recall the differences in the trees predicted by the Multi-regional and Recent African Origin Hypotheses.

Here are some features of the tree which you need to consider:

  • Are there several deep divergences in the tree, or just one?
  • For the samples from each geographic region, how deep in the tree is the most recent common ancestor?
  • Do the samples from each geographic region have the same amount of genetic variation?
  • Do the African samples show a recent or an old degree of relationship with samples from other regions?
  • For each non-African region, is there a tendency for them to have a single, or closely-related set of ancestors, or were there multiple lineages among the ancestors?

Which hypothesis, Multi-regional or Recent African Origin, do you think is supported by this tree?

Age of Most Recent Common Ancestor
An important factor to be considered in deciding between the two hypotheses is the age of the most recent common ancestor of humans. The Multi-regional Hypothesis predicts an ancient ancestor, occurring about a million years ago, while the Recent African Origin Hypothesis predicts a much more recent common ancestor.

The age of the common ancestor of humans can be estimated using the molecular clock. Certain types of genetic changes accumulate at an approximately constant rate. We can estimate that rate by comparing organisms which have been separated for a known period of time. When we observe genetic differences between two other organisms, we can estimate the date of their common ancestor as

Date of common ancestor = Observed genetic difference / Rate of Genetic change

Wilson had earlier estimated that the rate at which RFLP genetic differences accumulated in human mtDNA was 2 - 4% per million years.

Look at your tree and try to find the common ancestors of the clades listed in the table below. Use Wilson's estimate of the rate of genetic change to infer the age of modern humans. What date do these results indicate for the origin of non-Africans?

  Common
Ancestor
Observed
Differences
Hi Rate
(4%/My)
Lo Rate
(2%/My)
  Clade of all humans 0.57%
________
________
  Clade containing all non-Africans 0.45%
________
________
  Oldest Clade containing no Africans 0.43%
________
________

 

Why would we estimate these dates using both the high and low rates of genetic change?

Which hypothesis, Multi-regional or Recent African Origin, do you think is supported by these ages or dates?