The photographer Rachel Sussman has been traveling the world to take pictures of the oldest living organisms on our planet. She described her journey in this TED talk, and now, at last, she’s created a gorgeous new book, The Oldest Living Things in the World, published by the University of Chicago Press.

Rachel asked if I would write an introduction to the book. After contemplating her photographs and thinking about what these strange Methuselahs mean for us and for science, here’s what I wrote:

HOW LIVES BECOME LONG

It is easy to feel sorry for the gastrotrich. This invertebrate animal, the size of a poppy seed and the shape of a bowling pin, swarms by the millions in rivers and lakes. After it hatches, it takes only three days to develop a complicated body, complete with a mouth, a gut, sensory organs, and a brain. Having reached maturity in just seventy-two hours, the gastrotrich starts laying eggs. And after a few more days, it becomes enfeebled and dies of old age.

To squeeze a whole life into a week seems like one of nature’s more cruel tricks. But that’s only because we are accustomed to measure our lives in decades. If the ancient animals and plants featured in this book could look upon us, they might feel sorry for us as well. We humans marvel at the longest-living human on record, Jean Calment, who lived from 1875 to 1997. But for a 13,000-year-old Palmer’s oak tree, Calment’s 122 years rushed by as quickly as a summer vacation.

Palmer’s oaks, gastrotrichs, and all the species in between are the products of evolution. The head-swimming diversity of life is joined in an evolutionary tree made up of tens of millions of branches. And one of the most spectacular of that diversity’s dimensions is longevity. If natural selection provides Palmer’s oaks with millennia, why does it only spare a gastrotrich a week of existence?

Starting in the 1960s, evolutionary biologists have searched for an overarching explanation to account for all the different ways to grow old. The best-supported ones so far are variants on the old truth that a jack-of-all-trades is a master of none. An organism can collect a finite amount of energy, whether it’s a lion killing gazelles, a tulip capturing sunlight, or a microbe breathing iron at the bottom of the sea. It can use that energy to grow, to produce offspring, to defend itself against pathogens, to repair damaged its damaged molecules. But it has a limited budget. The energy spent on one task is energy that can’t be spent on others.

Molecular repair and pathogen defense are both good ways to live longer. But a long-lived organism that produces few offspring will not pass on many copies of its genes to future generations. The organisms that will succeed are the ones that do a mediocre job of keeping their bodies in order, leaving more energy for making babies.

This balance goes a long way to explaining why some species live long and some short. It may give scientists clues to how we humans might battle the burdens of aging, such as Alzheimer’s disease. But this balance is only part of the answer to why things live as long as they do. The environments in which species live may also be a part of the answer. In some places, life may simply run slower. Some lineages may evolve a way out of the binds that tie most species. They may escape the trade-offs that come with channeling energy in one direction or another, and be free to live longer.