Scientific American - USA (2022-02)

48 Scientific American, February 2022

The propensity of transposons to multiply means that

all genomes have a tendency to expand over time. Like

stuff in a garage, DNA will accumulate to fill whatever

space is available. It is the pressure of natural selection,

penalizing the host when its genome gets too big, that

maintains the genome at a certain size in most species.

There are patterns to that size, Gregory says. The DNA

load that a species can tolerate depends on its speed of

development, its metabolic rate and the way it lives.

Birds, with their fast metabolism and energy­

demanding flight, simply can’t manage a lot of bulky

DNA. Their genomes are smaller than those of most

mammals, ranging from 0.89 to 2.11 gigabases—fewer

than the 3.06 gigabases for humans. Among mammals,

19 of the 20 smallest genomes belong to bats, which face

challenges similar to birds’.

We humans fall in the middle of the mammal pack,

which probably reflects several competing factors. We

develop slowly, taking almost 20 years to reach adult­

hood, which implies that we should have significant

capacity for carting around extra DNA. But the size of

our genome may sit on a razor’s edge, kept in check by

another critical factor: the brainpower that we depend

on for survival. Suzana Herculano­Houzel, a brain sci­

entist at Vanderbilt University, thinks humans and other

primates owe their outsize intelligence to the fact that

their nerve cells are relatively small, allowing us to cram

more of them into our cerebral cortex. If her theory is

THE WATERDOG
compensates for
its frail heart and
lungs by breath­
ing through gills.
Despite being
riddled with
junk DNA,
the salamander
has found
ways to survive.