First, a few 'random' observations:
I was so disappointed that the enhanced squid didn't turn out to be the dominant species in the Manifold at the end, especially after the teaser when Baxter wrote that they were inherently better fitted for space travel than humans. But he even wrote "Humans are the most important sentient creatures who have ever existed, or will ever exist", just to take away any hope I might have that it was going to come. What a missed opportunity, it would have been so much more interesting that way.

"His elaborate [South African] accent, forever linked to a nightmare past, made her skin prickle." The book was written in 1999. At that time, the accent indeed made me cringe, but encouragingly, it is not 'forever', it is now not a problem.

"This has been a good place to cycle,” Anna said dreamily. “Of course that’s why we built it this way.”
“You built a toy universe so you could ride your bikes?”
She grinned at Emma. “If you were ten years old and could build a universe, what would you do?”"
Nice thought, but why ever would Stephen Baxter think that having fun riding bikes is only for children?!

"Suppose you have a lily, doubling in size every day. In thirty days it will cover the pond. Right now it looks harmless. You might think you need to act when it covers half the pond. But when will that be? On the twenty-ninth day."
Who would have thought that with the pandemic, we would all have heard such illustrations of just what exponential growth means multiple times?!

The book is a bit of a mixed bag. Some of the "real" (according to the acknowledgments) science is rather dubious. For instance, the probability argument makes no real sense, especially to anyone who knows the first thing about population dynamics. The section on flitting between parallel universes is boring. But in general, it was a great story, enjoyable reading and not too dated.