what this "common ancestor" looked like, or if it even existed at all! If you're going to say that this "common ancestor" just appeared, then why not simply believe that God created humans?
The
most recent common ancestor of humans and chimps likely looked a little like either - tailless, hairy, probably walking sometimes on two feet and sometimes knuckle-walking. Likely they were decent tree-climbers. They almost certainly had small brains - ape-sized ones. They had to eat fruit to get vitamin C, because a gene required to make it was inactivated in their species. They could get gout, because a gene required to oxidize uric acid was inactivated in their species.
They most certainly didn't "just appear." They had ancestors, too, just like you and I do. As you dig up older and older fossils, these ancestors get smaller and less ape-looking. They would be described by a zoo-goer of today as more like a monkey or a lemur, perhaps. Go back to fifty million years ago and you'd say "squirelly-looking". 340 million years and you'd swear it was a toothy salamander. All these critters bred with critters that looked almost like them - at least as much as I look like my wife. All of them had kids that looked almost like them - as much as my kids look like me. But they weren't identical. Five hundred generations down from some ancestor in the line that led to you and me, you might have occasionally found a later ancestor that looked enough different to make you say, "Hmmm. That may not be quite the same critter as that last one I examined! The zygomatic process isn't quite the same...."
The reasons I don't "simply believe that God created humans" are that the rocks and fossils and DNA don't lie. There are bucketfuls of evidence for our common ancestry with not just chimps, but with all life on earth. There's not a thimbleful of evidence for any god.