dmehling

Is it true that radioactive elements contained within igneous rocks do not begin to decay until the magma from which these rocks were formed begins to cool? If so, why does the decay not begin until the magma cools?

I get the point, sort of. But I don't see the logic. As an example, consider the cat family. Lions, tigers, leopards, cougars, cheetahs, etc. all came from one catlike ancestor. By what principle can we think that that ancestor must be extinct? Not all of its offspring will necessarily be transitioning into other species, and so we could expect this species to continue existing. But while some of the descendents remain the same, other descendents could be evolving into new species. Maybe this should not be a common occurrence, but it would not seem too much to think that we could find at least one example of this in nature. Having said that, the more important question I was asking is if we have specifically identified any common ancestors in the fossil record. Is this a determination that could actually be made, or do the fossils simply not provide enough information about the creature in order to make more detailed comparisons with modern-day species? Sorry I keep posting the same thing… My Internet connection keeps going out. Didn't realize it had already gone through.

I get the point, sort of. But I don't see the logic. As an example, consider the cat family. Lions, tigers, leopards, cougars, cheetahs, etc. all came from one catlike ancestor. By what principle can we think that that ancestor must be extinct? Not all of its offspring will necessarily be transitioning into other species, and so we could expect this species to continue existing. But while some of the descendents remain the same, other descendents could be evolving into new species. Maybe this should not be a common occurrence, but it would not seem too much to think that we could find at least one example of this in nature. Having said that, the more important question I was asking is if we have specifically identified any common ancestors in the fossil record. Is this a determination that could actually be made, or do the fossils simply not provide enough information about the creature in order to make more detailed comparisons with modern-day species?

Are there any currently living species that could be considered a common ancestor to any other species? Also, have any actual common ancestors in the fossil record been identified? If not, is it because the concept of a common ancestor is more of a theoretical determination? In other words, we know that evolutionary theory requires common ancestors but we have no way to make an absolute identification and can only suppose that a particular fossilized creature could potentially be a common ancestor to something else?