What is the evidence, that all vertebrate land animals proceed from a single marine vertebrate lineage?

So why only one and not, say, two, three or seven very similiar lineages? It is difficult for me to think that vertebrate animals accomplished marine to land only once but managed land back to water on several occasions (whales, penguins, seals, etc..). Thank you.


A recent discovery, recent could be anytime in the last couple of years to me. Has thrown into question what and how many, up and crawled out of the swamp. That eventually turned into us. So you are not the only one now asking that question.


I have never heard of just one marine species is the ancestor of ALL land animals. Where did you get this idea from? Oh, I took biology both in high school and in college.


All land vertebrates (reptiles + amphibians + birds + mammals) are known as tetrapods. It is definitely true that some vertebrates (e.g. mudskippers) have become land animals, moving around on beaches with their fins. Mudskippers, however, are ray-finned fishes, not tetrapods. Their fins have a large number of thin bones. In contrast, all reptiles, amphibians birds and mammals have 4 limbs and the same arrangement of bones that can also be found in lobe-finned fishes. That is strong evidence that all land vertebrates descended from lobe-finned fishes. Therefore all land vertebrates are descendants of lobe-finned fishes that had 4 limbs. The question therefore is whether one lobe-finned fish with 4 limbs or several of them could have been the common ancestor of all land vertebrates. The fossil evidence suggest that there were more than one lobe-finned fish with 4 limbs. For example Ichthyostega and Acanthostega were lobed finned fishes with 4 limbs and finger and toe bones. These 2 fossils have more than 5 fingers, but all known tetrapods have a maximum of 5 fingers and toes. There are some that have more fingers and toes, e.g. ichthyosaurs, but these additional fingers appeared to have evolved by duplicating some of the 5 existing fingers. All other known reptiles have but 4 limbs and 5 fingers and 5 toes (not counting those that have evolved fewer, such as snakes, legless lizards and amphisbaenians). For this reason, the 4 limbs, five fingers and 5 toes are evidence that all amphibians, reptiles, birds and mammals are descendants of a single common ancestor. It would be unlikely that if more than one ancestor was ancestral to tetrapods, that they all converged later on the same trait of 5 fingers. That means Ichthyostega and Acanthostega are unlikely to have given rise to different lineages and both lineages later evolved to have exactly 5 fingers and 5 toes, no more and no less. Additionally the amniotic membrane, which is found in all reptiles, birds and mammals is evidence that they all have a single common ancestor that evolved the amniotic membrane. Could there have been other land vertebrates that evolved from lobe-finned fishes? Yes it is possible but if they did exist, they are evolutionary deadends because the living tetrapods, due to the pentadactyl hands and feet, are almost certainly the descendants of the same common ancestor.


We all appear to be pentadactyl tetrapods. Except for mudskippers. Sort of. so i herd you liek mudkipz


I don't know but usually the original evidence is just the contents of the fossil record and tracing various species through time. More recently DNA evidence reveals a lot about ancestry, particularly the way in which it is nested. All we can ever say is that currently there is good evidence it happened once but no supporting evidence to show it happened more than once - unless there is some now.