The evolution of whales and dolphins

Evolution is the change in the inherited characteristics of a population over time by a process called natural selection which may result in the formation of a new species.

Natural selection is a process by which the most suitably adapted variants in a population survive and pass on their genes to the next generation.

Evidence for evolution can be found in the fossil record.

This is all very well but how do these bland statements actually relate to the evolution of whales? 

It all starts with a mangy looking dog that lived 55 millions years ago. The dog is called Pakicetus.  It belongs to the order Artiodactyle or the even toed ungulates. This is why that mangy old dog is often called “a wolf with hooves” or, quite literally, a wolf in sheep clothing. Pakicetus fossils were discovered in Pakistan.  Now 55 billion years ago Pakistan formed part of the coastline of the Tethys Ocean. Pakicetus was thought to live on the coastline. 

At first scientists made no link between Pakicetus and whales until they looked closely at the the fossil. In Pakicetus one of the bones near the inner ear was thickened.  Now the only modern organisms that also show this thickening are whales and dolphins.  So the fossil record is able to link extinct organisms with modern organisms and suggest an evolutionary connection.

Why would this curious thickening be of any use to our mangy old dog? Well the thickening allows an organism to hear better under water.  So Pakicetus, when looking under water, would have been able to hunt better. The fossil record also tells us what they ate.  By looking at the wear on the teeth scientists showed that their diet was fish.

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The word Cetacean (whales and dolphins) comes from the Ancient Greek - Ketos. Ketos means great fish

Next in the evolutionary line, about 46 million years ago, comes Ambutocetus. Ambutocetus is definitely living in the water. Studies of oxygen isotopes in their teeth suggest that they lived in salt and freshwater.  This means they could have lived in an estuary.  The fossils show their limbs still supported their weight on land.  They did not move very fast in water and were most likely to ambush their prey much like modern crocodiles. Though they may have hunted like crocodiles they swam like otters with their hind limbs and tail. 

Cetaceans are unrelated to other sea mammals such as seals, walruses and manatees

The next milestone in the evolution of whales was Rodhocetus.  The fossils show that the hind legs were too weak support the weight of the animal outside of water.  It is also thought that they lived in a saltwater environment, and like modern whales drank seawater.  They may have been like very large seals.  However unlike seals, Rodhocetus still had a tail. Also of interest are the nostrils of Rodhocetus.  They are further back in head.  This is the start of the migration of the nostrils to the top of the head to form the blowhole in whales.  

Humans have hunted whales since 6000 BC

The first true looking whale was thought to have evolved about 44 million years ago. Dorudon had no external limbs but like modern whales, its skeleton has vestigial limbs present. Once again the nostrils have moved further back up the skull.  The Dorudon vertebral column demonstrates a close similarity with modern day dolphins and whales.  This had led scientists to speculate that Dorudon had evolved tail flukes to help with swimming.  Unfortunately tail flukes are soft tissues and so do not leave behind fossil remains. How do these structure develop? Rhodocetus are unlikely to have had flukes, so the structures seemed to have appeared by magic. 

New phenotypes do not appear by magic but by mutation.  Most mutations in the DNA are actually lethal and will kill the organism while it is developing in the womb.  Many mutations in the DNA actually cause no change.  But sometimes a mutation may actually cause a beneficial change.  In the case of the tail flukes it would enable the organism to swim faster.  This means the organism is now able to escape from danger and survive.  There where very many big sharks back in past.  As a result the whales with tail flukes would be more likely to survive and pass on their mutated tail fluke genes.

Now we come to the part of the story where are ancient whales diverge into two groups.  The dolphins and the whales. By this stage the blow hole is at the top of the skull. Both groups had evolved echolocation at this point.  When I was young I always wondered why dolphins needed echolocation when they were always swimming in crystal clear tropical seas.  But the tropicals seas are where the dolphins ended up. They started in rivers like the Amazon and the Nile.  These rivers are very silty and it is impossible to see.  As a result those organisms who developed echolocation would be at an advantage over those that did not. 

So evolution can be tracked by studying fossils from the past.  The evolution of the blowhole in whales and dolphins is an example of this.  By comparing fossils with modern organisms scientists can follow the changes over time for particular features.

Fossils can also be used to track changes in organisms over millions of years and link those organisms together.  Scientists can plot these changes and show the relationships between these organisms using diagrams called evolutionary trees.  These allow scientists to study relationships between modern organisms and classify them correctly.  Because the fossil evidence suggested a common ancestor, 55 million years ago, for whales and hippopotamuses scientists were able to group them together in the same Order (Artiodactyle).

Care must be taken with the fossil record.  The fossil record only preserves some extinct organisms. If you look carefully at the fossils on the diagram you will notice that on many occasions a complete skeleton is not found.  The red coloured bones are missing from the fossil.  As a result scientists make assumptions to fill in the gaps.  When they do this they publish their results for other scientists to read.  This is called peer reviewing.  It allows all scientists to study the data and challenge a hypothesis if required.  

The information for this article has been gleaned from a magnificent piece of work called: 

From Land to Water: the Origin of Whales, Dolphins, and Porpoises by J. G. M. Thewissen et al.

The information in that paper has been distilled and simplified to help bring some colour into the GCSE syllabus.  There is is much more detailed evidence about the evolution of whales in this paper.  It is highly recommended that you read or even just scan the paper to see the wealth of evidence that scientists have collected to allow them to draw the evolutionary tree of whales.