The foods our ancestors consumed don't really exist anymore. No, really, that broccoli you're eating didn't exist back in their times, and the ancestor of the broccoli plant that they ate bears little resemblance to the vegetable today. They didn't eat fatty cuts of meat, they ate super-lean meat when they could catch it. They didn't eat onion and garlic fried in olive (or coconut) oil. If they found carrots, they weren't anywhere near as large, sweet, or nutrient-rich as the ones you buy in a supermarket. Here's an archaeologist talking about it.
So given that we can't eat the diet our ancestors consumed, why discount an enormous range of foods that we have created because some others we have created (through very selective breeding) evoke some "natural" ideal? It's not difficult to argue that eating excessive quantities of deep-fried starchy food is bad for you, but that's not cause to throw out grainy breads as well. You can try arguing that coconut oil is good for you, but there isn't enough research on the subject available to conclusively decide one way or the other yet - or we would've decided already.
The argument that you can eat "what we evolved to eat" is an appeal to nature, essentially. It's not possible to eat what we ate 150,000 years ago without putting a lot of effort into finding some really crappy meals. Paleo is a fad diet which may not be harmful, but its rules are as arbitrary as any others.