Assumption: Marine mammals (e.g., cetaceans) evolved from terrestrial ancestors.
Fact: Most aquatic vertebrates have gills, but the cetaceans (e.g., whales) have lungs.
Question: Why whales do not have gills?
[I’m a Brazilian biologist and writer. I write about science (mainly about population biology) and would like to know the opinion of colleagues from any field of scientific knowledge (and from other countries).]
See also Trees in Sahara (https://www.researchgate.net/post/Trees_in_Sahara).
The simple answer is that marine mammals are not fish! But also, it depends what you believe. Like you said, evolution is an assumption, so there can be other possibilities such as creation :)
Hello Felipe;
Let's start by getting the vocabulary of evolutionary biology straight.
1. Your statement "Marine mammals....." isn't an assumption. It is a conclusion derived from a vast body of evidence. There are mammalian structures that do derive embryologically from gill structures. Mammalian ears are derived from the same emb. structures that give rise to the spiracles of many fish. The mammalian hyoid apparatus is homologous to the several gill arches of fish.
Trace the gill arches of Crossopterygian fish to amphibians, reptiles to mammals and you can figure out the phylogeny of tetrapods. Then look at the fossil evidence and you will find that it corroborates the embryological evidence.
So, marine mammals respiratory apparatus is an historical consequence of their terrestrial origins.
2. Andrew, in his reply, describes evolution as an assumption. It is not! Assumptions are statements that are taken to be true without evidence. That definition hardly characterizes the body of evidence that supports evolutionary theory.
Cheers, Jim Des Lauriers
I thank to James and the others colleagues for the comments.
And I also apologize for any misunderstanding that my words of yesterday can be generated.
Evolution is a fact, and I think most of us here would agree with this. However, the word evolution can be interpreted in three different ways (Ruse 1985): (1) the fact of evolution; (2) the evolutionary history (phylogeny) of lineages; and (3) the process of change within populations.
The discussion that I proposed involves only items 2 and 3. So, what I sorted as an assumption – and not as a fact – was the habitat (and habits) of the immediate ancestors of cetaceans. Most scholars (e.g., Gatesy et al. 2013) seem to defend the idea that these ancestors would be animals of terrestrial habits, but there are still alternatives hypotheses valid and these would include amphibious or aquatic clades.
Cited references
+ Ruse, M. 1985. Taking Darwin seriously. Oxford, Blackwell.
+ Gatesy, J & more 7. 2013. A phylogenetic blueprint for a modern whale. MP & E 66: 479-506.
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