What topic are the man and woman discussing?

M: I really appreciate your filing me in on yesterday’s lecture.
W: No problem. I thought you might want to go over it together. And anyway, it helps me review. Hope you’ve feeling better now.
M: I am. Thanks. So, you said she talked about squid? Sounds a little strange.
W: Well, actually, it was about the evolution of sea life—a continuation from last week. The octopus and the squid descended from earlier creatures with shells. They survived by shedding their shells—somewhere between 200 and 500 million years ago.
M: That’s a pretty long span of time.
W: I know. That’s what she said, though. To be precise: "Exactly when they emerged is un- certain... and why is still unexplained."
M: Some squid are really huge. Can you imagine something that big if it still had a shell?
W: Actually, it’s because they lost their shells that they could evolve to a bigger size.
M: Makes sense. But some are really huge. I’ve read about fishermen that caught squid that weighed over a ton. Did she talk about how that happens?
W: Not really. But she did mention some unusual cases. In 1933 in New Zealand they caught a squid...let’s see here... it was twenty-two yards long. Its eyes were eighteen inches across. Can you imagine?
M: Reminds me of all those stories of sea monsters.
W: Dr. Simpson thinks there are probably even larger ones that haven’t been found because squid are intelligent and fast so they can easily get away from humans. Maybe some of those monster stories are true.