Children of Time is a 2015 science fiction novel by Adrian Tchaikovsky.
In the distant future, humanity seeks to create new habitats for itself on distant planets, terraforming them and seeding them with life. Dr. Avrana Kern is heading one such project, orbiting the tentatively named "Kern's World", where the plan is to release monkeys 
infected with a nanovirus that will accelerate their evolution. Through an act of sabotage from an anti-technology group that has also destroyed much of Earth, the monkeys are never released, and the virus instead infects a species of spider, Portia labiata. The book follows the evolution of the spiders and their eventual civilisation, as well as a remnant of humanity that fled to Kern's World hoping to find paradise.
also children of ruin and children of memory, the sequels, are really good
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That person in particular looks way closer <16 in terms of BMI (as in they're not being truthful about their actual weight) and is gonna be less fertile due to issues relating to malnutrition and hormone interruption particularly if they have a history of anorexia. Also even if the end up carrying a child, the kid ends up with a crazy growth restrictions unless the parent is willing to eat more than usual for them. You do eat for "two" but it's like an extra bagel a day - and to someone with an ED, that might be a thing they're unwilling to do. Pregnant people sometimes continue using some intense drugs like meth or opiates, it's very possible to continue having an ED.
For the others in the comments, they might be truthful or not. Everybody's bodies look different. I wish I could say "don't get hung up on it" but I know that's hard. Do your best to stick to a healthy BMI, but BMI isn't the end all be all anyway. As a woman, including a trans woman on HRT, a healthy body fat % is actually higher than pre hrt. Do your best not to compare to pictures and focus on what is medically healthy, pictures can lie as in misreport an actual weight or they can be modified by things like photocopy. Your wife says you're pretty how you are, right? That means something too.