Pakicetus

The Department of Geology

The School of Earth, Society, and Environment

College of Liberal Arts and Sciences

Virtual Field Trip Home

Correct! The teeth of ancient whales (Archeocetes) and Mesonychids are similar. Based on that trait, scientists believed that ancient whales evolved from Mesonychids some time during the Eocene. Open the packet labeled '2' and check the contents. You should have: A Pakicetus 'fossil' 5 DNA sequences (Right Whale, Giraffe, Hippo, Cow, and Camel) In 1983, all we had were these primitive whales and Mesonychids, with a big gap in between. This year, paleontologist Philip Gingerich was searching in Eocene deposits in Pakistan, and found the skull of an amazing fossil. It had teeth like the Dorudon whale, with whale-like ear bones and other features, but it was much older (50 mya), and there were indications that it had four legs. He called this Pakicetus, so place your Pakicetus strip on your timeline at 50 mya. At first, only the skull of Pakicetus was found (this will turn out to be important later!). Knowing that whales must have evolved from a terrestrial ancestor and that Pachyaena was almost 55 million years old, Gingerich predicted that a close whale ancestor would be slightly younger than Pachyaena. He looked at rocks that were 50 million years old and found that fossil. Around this time DNA sequencing allowed molecular biologists to determine which living animal is the closest relative to whales. Remember that DNA has 4 'base-pairs', 'A', 'T', 'G', and 'C'. The closer two organisms are related to each other, the fewer differences there will be in DNA sequences. Take the DNA strip for the Right Whale, notice below the DNA sequence is a box where you can put the other species. Place the other species (the Giraffe, Hippo, Cow, and Camel) next to the Right Whale and count the number of differences in the DNA for each of them. Using this information we can create an 'evolutionary tree', which is a diagram that shows us how organisms are related to each other. They look like this:

In this diagram there are three 'species', species 'A', species 'B', and species 'C'. Each of these is placed at the 'tip' of the tree. Connecting each species is a 'branch', and branches meet at the 'node'. We can tell which species are more closely related to each other by counting the number of nodes between them. For example, to get to the node between 'A' and 'B', put your finger on 'A' (or 'B', it doesn't matter). Trace the branch down to the 'node' that connects them, which in this case is the 1st one. So 'A' and 'B' are one node away from each other. To get to the node between 'A' and 'C', we have to go 2 nodes. So from the information in this diagram, we would say that 'A' and 'B' are MORE CLOSELY RELATED to each other than either is to 'C'. Now make an evolutionary tree of the Right Whale, Giraffe, Hippo, Cow, and Camel, based on the number of differences in their DNA (knowing that the fewer the differences, the more closely related two animals are).

Which animal is most closely related to the Right Whale, the Giraffe, Hippo, Cow, or Camel? Click Here to Answer

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