1428 17 DECEMBER 2021 • VOL 374 ISSUE 6574 science.org SCIENCE
IMAGES: (TOP TO BOTTOM DEVLIN A. GANDY; LAWRENCE LIVERMORE NATIONAL LABORATORY2021 BREAKTHROUGH OF THE YEAR^ | RUNNERS-UP
Ancient soil DNA comes of age
DNA from fossils has transformed the study of human and animal evolution, reveal-
ing unknown relationships, tracing early migrations, and exposing ancient interspecies
mating. Yet for humans, the entire field depends on just 23 archaic genomes, 18 of them
from Neanderthals. Recently, scientists unlocked a much larger trove of ancient DNA:
from the soil of cave floors. This year, for the first time, cave dirt yielded DNA once
housed in the nucleus of human cells, and researchers used such “dirt DNA” to re-
construct the identity of cave dwellers around the world.
The new work borrows from the study of environmental DNA from living species. To
find out which organisms inhabit lakes, forests, and other places, scientists collect the
free-floating DNA they shed into air, water, and soil. By 2003, evolutionary geneticists
showed discarded DNA could persist for thousands of years. It was used by researchers
in 2015 to help reconstruct entire ancient ecosystems, even in the absence of fossils.
But much of that DNA comes from mitochondria, the cell’s power plants, which store
tiny snippets of genetic material. Thanks to new techniques, scientists can now comb
ancient soils for nuclear DNA, which carries the full instructions for life.
This year, scientists used nuclear DNA to chart the human and animal occupation
of three caves. In Spain’s Estatuas Cave, nuclear DNA revealed the genetic identity and
sex of humans who lived there 80,000 to 113,000 years ago, and suggested one lineage
of Neanderthals replaced several others after a glacial period that ended 100,000 years
ago. In 25,000-year-old soil from Georgia’s Satsurblia Cave, scientists found a female
human genome from a previously unknown line of Neanderthals, along with the genetic
traces of a bison and a now-extinct wolf. And by comparing 12,000-year-old black bear
DNA from Mexico’s Chiquihuite Cave with that of modern bears, scientists discovered
that after the last ice age, the cave bears’ descendants migrated as far north as Alaska.
Techniques for extracting and sequencing nuclear DNA from ancient soils are still
improving. As they do, researchers hope to answer even more questions about the rise
and fall of ancient species. —Elizabeth PennisiFusion’s day in the Sun?
Is fusion energy about to overcome its reputation as a field that
promises the stars but never delivers? In an August result that sur-
prised the researchers themselves, the U.S. National Ignition Facility
(NIF) produced a fusion reaction that came tantalizingly close to
reaching official “breakeven,” the point at which a reaction produces
more energy than the laser energy needed to kindle it.
Fusion, which powers the Sun and other stars, has long been seen
as a solution to Earth’s energy problems. But achieving the pressures
and temperatures required—10 times as hot as the Sun’s core—is no-
toriously difficult. Many efforts cage a superhot plasma in a magnetic
field; NIF uses a pulse from the world’s highest energy laser to com-
press a peppercorn-size capsule of the hydrogen isotopes deuterium
and tritium. Earlier this year, that method generated 170 kilojoules of
fusion energy per shot—far short of the laser input of 1.9 megajoules.
But in an 8 August shot, that yield surged to 1.35 megajoules.
Researchers think it’s the result of a burning plasma, meaning the
fusion reaction generated enough heat to spread through the com-
pressed fuel like a flame. The result has not been scrutinized by a
peer-reviewed journal, but it was presented at the American Physical
Society’s Division of Plasma Physics conference in November.
Now, the team is trying to understand the shot’s high yield and
figure out how to tweak starting conditions to do even better, by us-
ing larger or smoother fuel capsules, more even layers of frozen fuel,
or higher quality laser pulses. They’re also making efforts to replicate
the shot: One attempt in October reached 430 kilojoules, and a No-
vember shot hit 700 kilojoules. Attempts will continue into 2022.
As NIF edges toward breakeven, private fusion projects are upping
the pace. Several predict they will generate energy long before the
ITER reactor, a $25 billion publicly funded magnetic fusion effort.
This year, Commonwealth Fusion Systems and Tokamak Energy
claimed progress with high-temperature superconducting magnets.
And General Fusion and TAE Technologies, which use pistons and
particle beams, respectively, are planning energy-producing demon-
stration power plants they say will switch on in 2025.
Whichever approach reaches energy gain first, formidable chal-
lenges in materials science and engineering remain before fusion can
To produce NIF’s fusion shot, 192 laser beams converged around a tiny fuel pellet. become a practical power source. —Daniel Clery