paleontologybut large-scale genome writing …jast239/courses/biogeo/readings/extinctions.pdflian...
TRANSCRIPT
Where humans tread, extinction fol-
lows. So researchers have thought
for decades, persuaded by ample
evidence that—in the last few
thousand years, at least—the ar-
rival of humans almost always
brought overhunting, habitat destruction,
or invasive species that killed off native crea-
tures. Nowhere did the maxim seem truer
than on islands, with their limited resources
and naïve prey. But evidence presented by an
international group of paleontologists and
archaeologists at a meeting here at Austra-
lian National University (ANU) last month
suggests that in the more distant past, the
story was different: When humans first
landed on isolated islands during the Pleis-
tocene, 10,000 years ago and more, their im-
pact was surprisingly light.
Few question the destruction wrought
by later colonizations, such as the arrival
of the Polynesians in New Zealand some
700 years ago, which precipitated the de-
mise of creatures including the giant flight-
less moa. But new evidence from Sri Lanka,
the Indonesian island of Flores, and else-
where persuaded many participants in the
workshop, called “First Contact: Impact of
Pleistocene Hominins on Island Ecosys-
tems,” that humanity’s early record hasn’t
always been so dismal. “I (and others) have
in the past argued that when humans first
reach islands, the extinction of endemic
animals is quick and certain,” archaeologist
Alan Simmons of the University of Nevada
in Las Vegas commented later in an email
to Science. “But it now appears that in many
cases, there was, in fact, a long co-existence
of humans with now extinct animals.”
The new insights are hard-won—and
preliminary. So far, only a few islands have
yielded enough artifacts and bones for re-
searchers to begin piecing together when
early humans arrived, what extinctions
occurred, and whether people had a hand
in them.
In a few cases early humans do appear to
be guilty. On the island of Cyprus, a cache
of dwarf hippo bones found 20 years ago
by an 11-year-old boy at a site called Ak-
rotiri Aetokremnos suggests the species
went extinct on the island about 12,000
years ago, shortly after people arrived.
The timing convinces Simmons that the
arrival of humans, together with a sud-
den cold, dry spell, killed off the hippos.
Although critics point out that the bones
lack the tell-tale cut marks of a human kill,
“you don’t need big tools to kill hippos,” he
said at the workshop. “These were naïve
animals, they had no predators and they
would have been easy prey for the ultimate
predators—humans.”
“That was one of the better lines of
evidence of extinction,” says ANU paleo-
ecologist Julien Louys, who co-organized
the meeting with ANU archaeologist Sue
O’Connor. But as researchers presented
case studies from other islands, Louys says,
“it became clear that it was more the ex-
ception, not the rule.”
By 100,000 years ago, for example, the
island of Flores already had human inhab-
itants: the hobbitlike Homo floresiensis.
Hanneke Meijer, a vertebrate paleontologist
from the University of Bergen in Norway,
has excavated animal bones from Liang Bua
cave, where the hobbit was found. At the
conference, she reported finding fossils of
2-meter-tall storks, dwarf elephants, scav-
enging birds, and other animals in the same
geologic layer that yielded the hobbit. A few
NEWS | IN DEPTH
674 19 MAY 2017 • VOL 356 ISSUE 6339 sciencemag.org SCIENCE
Island
extinctions
weren’t
inevitableDuring the ice age, human colonizers often coexisted with vulnerable island fauna
By April Reese, in Canberra
PALEONTOLOGY But large-scale genome writing faces
technical barriers. Although the cost of
DNA synthesis has decreased over the last
decade, it still hovers around $0.10 per base
pair. For now, companies supplying syn-
thetic DNA routinely produce fragments
of just 2000 to 5000 bases in length, notes
NYU Langone geneticist Leslie Mitchell.
Longer fragments, on the order of 10,000
bases, “come with a pretty penny price tag.”
As DNA strands get longer, they become
more prone to tearing when manipulated
outside of cells. At about 30,000 bases, “you
start to hit sort of the danger zone,” Mitchell
says. Yeast is an ideal host for bigger strands
because it can take up, assemble, and du-
plicate DNA much more quickly and read-
ily than human cells. Mitchell is part of an
ongoing international collaboration, born
from Boeke’s lab, to build synthetic versions
of all 16 chromosomes in the yeast spe-
cies Saccharomyces cerevisiae by overwrit-
ing segments of the native chromosomes
(Science, 28 March 2014, p. 1426).
For mammalian genome writing, scien-
tists will have to swap in large chunks of
DNA from yeast or another carrier without
disrupting the receiving cell’s vital func-
tions. At the meeting, Mitchell hinted at a
system her team is developing, which fuses
the DNA-containing yeast cell with a mam-
malian cell, targets the synthetic DNA to a
place in that cell’s genome with specially
engineered “landing pad” sequences, and
swaps it for the native sequence with help
from enzymes known as recombinases.
The meeting’s technical talks largely
left aside a central challenge for genome
writers: understanding natural DNA se-
quences well enough to improve on them.
“We complain about how much DNA syn-
thesis costs, but it actually costs a lot more
to design a million base pairs’ worth of
complicated circuitry,” Harvard synthetic
biologist Jeffrey Way said.
Geneticist Yasunori Aizawa at the Tokyo
Institute of Technology wants to probe the
importance of certain introns—DNA seg-
ments within genes that don’t code for pro-
teins but may still influence how other genes
are expressed—by designing DNA frag-
ments that lack them. William Efcavitch,
chief scientific officer of San Diego,
California–based Molecular Assemblies,
one of several companies developing new
synthesis methods, wondered to the techni-
cal working group whether GP-write should
first design a minimal human genome con-
taining only the necessary bits of code.
At that, Hessel’s imagination escaped
the confines of the project. “I think that’s
really interesting,” he chimed in. “I read a
science fiction book where one of the char-
acters was intron-free.” j ILL
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of those creatures, including the elephants
and the avian scavengers, later vanished.
Climate change seems a more likely cause
than hunting, though—H. floresiensis may
have been a scavenger rather than a hunter
and could easily have become prey itself.
“It is not unlikely … that storks would have
gobbled up a baby hobbit if it happened to
run by,” Meijer said.
Even when modern humans landed on is-
lands, they sometimes hunted in a sustain-
able way. In Sri Lanka,
Patrick Roberts of the
Max Planck Institute for
the Science of Human
History in Jena, Ger-
many, analyzed animal
bones found at archaeo-
logical sites between
3000 and 36,000 years
old. He found that three
species of monkeys—the
toque macaque, gray
langur, and purple-faced
leaf monkey—accounted for about 70% to
80% of human kills.
Monkeys are big, easy to find, and slow to
reproduce, which makes them particularly
vulnerable to overhunting, Roberts noted.
Yet all three primates are still around to-
day, and they’re still hunted by indigenous
people “with limited effects,” Roberts wrote
in a later email. (The gray langur and the
purple-faced leaf monkey are endangered,
but because of development and deforesta-
tion, not hunting, he added.)
Studies from other islands, including
Tasmania, the Philippines, the Channel Is-
lands, and Taiwan, also suggest Pleistocene
humans had a light ecological footprint,
leaving workshop participants to speculate
why. Smaller populations and simpler tech-
nologies than those of later arrivals might
have lessened their impact. The evidence
also hints that unlike later colonizers, the
early arrivals did not introduce invasive
species such as rats and dogs.
This emerging picture raises a new puzzle,
however. On continents such as North Amer-
ica and Australia, ice age immigrants are
blamed for widespread
extinctions of ice age
megafauna—think mam-
moths, giant sloths, and
giant kangaroos. “If we
cannot locate a definite
Pleistocene megafaunal
extinction on an island,”
says Roberts, “then what
does this mean for argu-
ments on the continents?”
Generalizing from the
studies so far is risky,
researchers emphasized. Environmental
conditions, the timing of settlement and ex-
tinctions, the technologies the new arrivals
brought—all vary from island to island, and
none of those factors is fully understood at
any site. But Louys finds a hopeful message
in the stories these islands are beginning to
tell. “Extinctions aren’t necessarily a char-
acteristic of our species—it’s something we
learned to do,” he said. “And if it’s some-
thing we learned to do, it’s something we
can learn to undo.” j
April Reese is a journalist in
Townsville, Australia.
SCIENCE sciencemag.org
“Extinctions aren’t necessarily a characteristic of our species—it’s something we learned to do.” Julien Louys, Australian
National University
Homo floresiensis shared the island
of Flores with giant lizards, dwarf
elephants, and other exotic species.
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(6339), 674-675. [doi: 10.1126/science.356.6339.674]356Science April Reese (May 18, 2017) Island extinctions weren't inevitable
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