June 2019, ScientificAmerican.com 1June 2019VOLUME 320, NUMBER 6Photograph by Floto + Warner
On THe COVeR
The fossil record shows that animal evolution
was ramping up millions of years before the
Cam brian explosion. Creatures from the pre-
ceding Ediacaran period were already evolving
into pre dators, making protective skeletons and
form ing reef ecosystems, among other develop-
ments that preceded Cambrian diversification.
Illustration by Franz Anthony.32
EVOLUTIONARY HISTORY
24 The Rise of Animals
New fossils and analyses of
ancient ocean chemistry reveal
the surprisingly deep roots
of evolution’s big bang—
the Cambrian explosion.
By Rachel A. Wood
NUCLEAR PHYSICS
32 The Deepest Recesses
of the Atom
Where do protons and neutrons
get their mass and spin?
We don’t know. A new particle
collider promises to look into
the sub-subatomic realm and
find answers.
By Abhay Deshpande and
Rikutaro Yoshida
ETHOLOGY
40 One Eye Open
Why dolphins, seals and other
animals developed the capacity
to sleep with half their brain
awake. By Gian Gastone Mascetti
C L I M AT E
46 Rough Weather Ahead
Climate change is making winter
storms and summer heat waves
worse. By Jennifer Francis
IMMUNOLOGY
54 Vaccines Reimagined
A controversial theory holds that
one immunization, given properly,
can protect against many diseases
besides its target.
By Melinda Wenner Moyer
S E C U R I T Y
62 Broken Shield
Systems designed to protect
against incoming nukes
could make us less safe.
By Laura Grego and David Wright
BIOTECHNOLOGY
68 All the World’s Data
Could Fit in an Egg
How DNA is used to store—and
generate—information at extreme
scales. By James E. Dahlman