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Explanation
The Joke
A person addresses "Dear Evolution" to ask why it is so hard being a mammal. Evolution (personified as a voice from above) says "Oh, that's easy" and launches into an explanation. It tells the person to consider that cockroaches have been around with the same body plan for over 300 million years, while mammals have been adapting for a much shorter time — so mammalian bodies are like getting "in a burgundy car redesigned to precisely adequate" levels. Evolution points out that humans have only been in their current form for about 300,000 years, and haven't really had time to properly enjoy features like walking, the first pair of shoes, or finding out about language. The final panel delivers the kicker: "Why do you think babies start life screaming?"
The Humor
The comic frames mammalian existence as a kind of rushed evolutionary prototype compared to more ancient organisms like cockroaches. The humor comes from Evolution being brutally honest and treating human biology as a barely adequate rush job. The final punchline — that babies scream at birth because even newborns can sense what a raw deal being a mammal is — gives a darkly comic "explanation" for a universal human experience. It's a classic SMBC move of using real scientific facts (cockroach evolutionary stability, relative youth of Homo sapiens) to build toward an absurd conclusion.