How do we know the cockroach is 250 million years old?

Dear Straight Dope: It is a frequently recited fact that cockroaches over 250,000,000 years old. How was this fact determined? What does the fossil record have to say about the estimable cockroach? Have they evolved in way, or are they basically the same now as they were in the Carboniferous era? Brooke, New York City

SDStaff Doug replies:

We know how old they are because of fossils. There are hundreds of different species of roaches in the world today, in several different families, and there are at least two other sub-orders of insects that are directly descended from them (termites and mantises), and possibly more. While none of those alive today is the same species as those in the Carboniferous, the fossils are more than similar enough to be recognizable as roaches. It’s not that amazing, really — virtually all of the modern groups of insects were abundant and diverse in the world as of about 100 million years ago. Bees and butterflies appeared just after the first flowering plants around that time; the one exception is the flea, which only evolved, from scorpionfly ancestors, after mammals appeared. In fact, if you exclude birds and mammals, most of the world’s present-day life forms (in general terms, not individual species) date back at least 100 million years. So 250 million is pretty good, but not quite as special as one might think.

SDStaff Doug, Straight Dope Science Advisory Board

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How do we know the cockroach is 250 million years old?

A STAFF REPORT FROM THE STRAIGHT DOPE SCIENCE ADVISORY BOARD