So who is the common ancestor and where is the fool proof evidence that it ever existed?
Honest question. I hear all of the time that we share a common ancestor, but nowhere has anyone showed any proof of that.
Evolution is based on anatomy, geology and genetics.
The geological record indicates that the further back in time you go, the simplier the organisms.
Go back far enough and you find only indications of bacteria- like organisms such as blue-green "algae" (really a bacteria with cholorphyll). As you progress through the geological record you find the remains of increasingly more complex organisms which have physical similarities to some organisms which preceeded them and some which succeeded them.
Organisms which have anatomical similarities are believed to be related - to have had common ancestors. When comparisons are done on the DNA of living organisms which are presumed to be related through evidence like this in the geological record, these relationships are generally substantiated. They are also substantiated through amino-acid studies on body fluids and tissues.
Look at snakes. Snakes have scales. So do lizards. Anatomically, they are very similar with some very significant difference. Snakes have no legs or pelvic or pectoral girdles in their skeletons. But SOME "prmitive" snakes like pythons, still retain very tiny remnants of rear legs. It is assumed, based on these factors that snakes evolved from lizards.
The earliest horse fossils are of small animals with several toes. As you move forward in the fossil record, you find the fossils of those horse are gradully replcaed with fossils of larger animals with reduced toes until the most recent horses have only a single one-hoofed toe.
Similar relationships have been observed in very many species of animals.
There is even embryological evidence. AS an embyo developes, it has certain features of a primitive nature which later disappear or altered into another form as the embryo grows larger.
Once it was thought that evolution preceeded in a rectilinear progression - a primitive species giving rise to a more advanced species, and so on until the final modern product was achieved. We now believe that evolution preceeds more like a growing bush. Some species evolve into current organisms, other became biological "dead ends". We don't really know for certain which of these primitive homids were our ancestors, but if they weren't directly ancestral to us, they most certainly were our genetic "cousins". A current debate along these lines involves Neaderthals and modern man. Most anthropologists believe Neanderthals were our gnetic cousins and di not evolve into modern man. They believe modern man evolved from some other ancestor which might have also been the ancestor of Neanderthals and later modern man exterminated or displaced Neanderthals. Some others believe that Neanderthals may have interbred with early Modern Man and contributed to our gene pool.
These studies are somewhat in a state of flux, but few if any serious biologists really question the basic premise that all living organisms are related and derived from some common ancestors.