#EvolutionEvents Episode 7 brings us to an amazing phenomenon of evolution, that sometimes sounds wilder than fiction!
First we introduce a term called “convergent evolution“. The principle is what it sounds like; evolution of different species moving towards a similar place and “converging”. One of the most widely known and talked about examples of this is found right in marine environments all around the world!
What could we be referring to?
As you can observe on this graphic, we have a fish as well as a humpback whale pictured. The humpback whale represents cetaceans, like other whales, dolphins and porpoises. Both of these creatures have a similarly shaped tail that propels them through the water, and some other similar features. Are each of their tails descended from the same ancestor who had a tail?
Without understanding convergent evolution, animals might be incorrectly sorted into family trees based on similar features or behaviours. This may create the assumption that fish and whales are more closely related to each other than a whale is to, for example, a hippopotamus (a true close relative). One may consider the streamlined body form of fish and cetaceans and the presence of swimming tails a “giveaway” to evolutionary origin.
Though, there has been suspicion into why whales are still so different from fish for a long time; Aristotle noticed that whales did not lay eggs as the majority of fish do. Hmmm….
So, the process of evolution can be much more complicated than what appears in modern animals. When we speak about two species, or types of animals living in the same environment, it is important to remember that their ancestors would have faced very similar challenges and requirements. For example, to live a fully aquatic lifestyle, a tail and a streamlined body shape are useful tools. Both fish and cetaceans utilize tails (albeit they move in different ways) to swim. However, fish and cetaceans did not alter the same “ancestral tail” to create the respective side-ways and vertical forms that we see with these animals today.
How did both of these tails appear?
It is true that all life did evolve from the ocean, but this does not mean that certain traits remained (like tail structures) and/or that unique traits would never show up.
Some of the most primitive fish were present in the ocean about 525 million years ago, with more modern forms appearing about 480 million years ago. The first life approached land from the Sea about 400 million years ago, in the form of prehistoric fish that actually crawled! These prehistoric crawling fish enabled the first tetrapods (four-limbed vertebrates, vertebrate means with a backbone). Tetrapods would give rise to many groups, including mammal-like reptiles called therapsids.
All mammals originated from these therapsids (meaning so did we) roughly 252-201 million years ago. Ungulate mammals were believed to have been the first precursors to whales. Remember when we discussed hippo and whale relations? Hippos are ungulates, like cows and giraffes, and are a living relative (but not ancestor) of modern whales.
So, in summary, these prehistoric fish gave rise to tetrapod land animals, thus providing the template for mammals to emerge and return to the sea to create whales.
What? Yes you read that correctly! The journey to become a whale has meant evolving in the Sea, leaving the Sea and returning back to the Sea. Breathing mechanisms have changed several times; gills to land lungs to lungs adapted for holding your breath for extended periods of time while under water.
Talk about a difficult puzzle to solve!
We can understand that the pressure of living an entirely marine life would have acted on the ancestors of whales. When we talk about natural selection, this notion becomes clearer. The mammals that were beginning to utilize the marine environment would have had a better chance of obtaining prey, thriving and surviving to reproduce, if they had traits more suited to living in/using the ocean, e.g. a more streamlined body for efficient swimming or a longer tail. Natural selection would have also acted on the ancestors of the first fish to create their streamlined form too, independently and earlier than these mammals.
As the ungulate “future whale” mammals spent time near ocean shores, 8 million years of selection would eventually enable a swimming mammal. The animal with the title of “first whale” was Pakicetus, a four-legged running cetacean who could also swim. A cetacean?!
Whales were only believed to have reached their massive modern sizes as of only 4.5 million years ago. To read more about whale-specific evolution, read some of our earlier blogs. Other animals, like seals and otters, have followed a similar, but not exact, pathway as cetaceans did from land to living a more marine life. Some cetaceans have even entered freshwater lifestyles, like river dolphins.
Both cetaceans and fish have diversified incredibly throughout the entire course of their evolution. Therefore, even though fish gave rise to mammals like cetaceans, many of their traits have not given rise to those that “look similar” on cetaceans. Cetaceans have gone through a rigorous and fluctuating evolution, in which many “land animal traits” (like a non-fish tail) have been repurposed for life in the Sea.
Article by: Alexa D. B.Sc., Naturalist with Five Star Whale Watching.