Woolly mammoth extinction due to warming climate
A team of British and Swedish
scientists just published a new study
indicating that changing climate --
not humans -- played a major role in
the extinction of the woolly
mammoth, Mammuthus primigenius.
Additionally, the team's analysis of
ancient DNA revealed that Eurasia
was colonised by woolly mammoths
that crossed the Bering Land Bridge
from North America around 66,000
years ago. They also identified a
previously unknown and genetically
distinct population of mammoths that
lived in Eurasia before they were
replaced by an influx of Siberian
mammoths approximately 33,000
years ago.
Below the jump, I tell you more about
the scientific team and how they
worked together to suss all this out.
Woolly mammoths, Mammuthus
primigenius, were very large and
hairy relatives to modern elephants.
These large mammals were specially
adapted to the cold, living only on
arid steppe-tundra of the far north.
They were widespread throughout the
Holarctic during the Late Pleistocene
(approximately 116 - 12,000 years
ago). Recently, a number of well-
preserved mammoths have been
found in the permafrost of Siberia
and Alaska (Beringia), providing us
with tantalising glimpses into the life
history and genetics of these animals.
For example, previous genetic studies
on Beringian mammoths identified
two deeply divergent lineages, dubbed
clade I and clade II, hypothesized to
have evolved in isolation on either
side of the Bering Strait (doi: 10.1016/
j.cub.2007.05.035 ).
Yet despite these remarkable insights,
we actually know surprisingly little
about mammoths, about the genetic
structure and population
demographics across their Holarctic
range, nor do we even know why
these animals ultimately went extinct
just 4,000 years ago. To investigate
these questions further, an
international group of scientists, led
by Eleftheria Palkopoulou, a doctoral
candidate at the Swedish Museum of
Natural History in Stockholm,
conducted an ambitious series of
studies where they obtained samples
from recently unearthed European
and Siberian specimens and
combined those data with previously
published DNA sequences to create a
mammoth-sized data set from more
than 300 individuals. By analysing
this dataset, the team reconstructed
the species' population history from
more than 200,000 years ago right up
until its extinction.
What did they do?
To carry out this study, the team
collected samples from 88 bone, tooth
and tusk specimens in specially
dedicated ancient DNA (aDNA) labs:
The DNA was purified and PCR-
amplified by Ms Palkopoulou and
Love Dalén, a paleogeneticist and
assistant professor at the Swedish
Museum of Natural History . The same
741 basepair (bp) fragment of
mitochondrial DNA (mtDNA) was
successfully amplified from 56 of the
88 specimens (16 more specimens
yielded a short 79bp fragment that
was informative for clade
identification only).
These newly amplified sequences
were aligned with the same regions
from previously published woolly
mammoth mtDNA sequences. These
provided a data set comprising
sequences from a total of 320
individual mammoth specimens,
ranging from the Late Middle
Pleistocene (more than 200,000 years
ago) up until the mammoth's
extinction, roughly 4,000 years ago.
What did they learn?
Ms Palkopoulou and Dr Jessica
Thomas, a postdoctoral researcher
who is currently based in the biology
department at the University of York ,
then did a lot of number-crunching
together. Based on their analysis of
the 320 mammoth mtDNA sequences,
the team identified 29 distinct
maternal haplotypes (figure S3;
larger view ):
As you can see in this data image
above, these 29 maternal mtDNA
haplotypes clump together into five
haplogroups (coloured clouds) and in
the figure below, we see that these
haplogroups cluster into one of three
distinct and highly diverged woolly
mammoth clades (figure S4; larger
view):
These clades correspond to small
pockets of favourable habitat, known
as refugia, where the mammoths
retreated during periods of climate
warming.
"This suggests that spells of warm
climate made the mammoth more
susceptible to extinction", said Dr
Dalén.
These genetic data suggest that there
were at least three separate
interglacial refugia during the
Eemian interglacial period, which
began about 130,000 years ago and
ended about 114,000 years ago. The
European mammoth (clade III)
distribution indicates that during this
time, there was an interglacial
refugium in Western Eurasia,
whereas the more restricted
distributions of clade II and clade I
mammoths imply there were two
additional refugia, one in northern
Siberia and the other in North
America, respectively.
But when did these genetic lineages
first split? Another way to ask this is
to estimate when the most recent
common ancestor of all the
mammoth clades existed. This
required yet more number-
crunching: this time, a statistical
model was used to analyse the
genealogy of the mtDNA region in the
database, tracking sequential changes
in this mitochondrial gene region
back in time to estimate when the
single ancestral copy existed (figure
2; larger view ):
Using coalescent simulations, the
researchers estimated the time
elapsed from when the three
populations expanded (figure 2a) and
split (figure 2b). The data analysis
estimated that the clade I woolly
mammoth population split between
North America and Eurasia
approximately 66,000 years ago
(figure 2c).
This population split also coincides
with the first time that sea levels had
receded enough for the Bering Land
Bridge to pop up during this
particular glacial period (these are
the time periods when the thin wavy
line dips below the heavy straight line
in figure 2d). The team inferred this
was also the timeframe when
mammoths moved westward into
Eurasia (this assumption is the most
likely explanation for these data,
although the researchers did not
explicitly test this hypothesis).
After the team mapped out the spatial
distribution of the radiocarbon-dated
and genetically analysed mammoth
specimens, a rather interesting series
of biogeographic snapshots emerged,
revealing that mammoths became
highly dynamic during the second
half of the Ice Age where some
populations expanded whilst others
disappeared completely (figure 3;
larger view ):
The team interpreted these data
images as follows:
following their range expansion
from North America to Eurasia, clade
I (purple dots) mammoths appear to
have lived alongside clade II (pink
dots) mammoths in Central and East
Siberia until the disappearance of the
latter
based on radiocarbon dates from
clade II specimens (pink dots), this
clade seems to have disappeared
approximately 45,000 years ago
clade I (purple dots) mammoths
expanded westward into Europe,
where they replaced the endemic
clade III (green dots) mammoths
clade III (green dots) mammoths
disappeared from the fossil record
approximately 34,000 years ago
clade I (purple dots) mammoths
seem to have first appeared in Europe
approximately 32,000 years ago
"This process culminated with a
severe decline in population size that
started when temperatures began to
increase at the end of the last Ice
Age", said Dr Thomas in a press
release.
What does this mean?
"We found that a previous warm
period some 120,000 years ago
caused populations to decline and
become fragmented, in line with what
we would expect for cold-adapted
species such as the woolly mammoth",
said Ms Palkopoulou in a press
release.
"[O]ur data suggests that the same
thing happened during the
penultimate warm period (an
interglacial some 120,000 years ago),
long before modern humans had even
left Africa", said Dr Dalén in email.
The patterns reported by this study
are similar to those described during
the same time periods for two other
holarctic species; the cave bear
(doi: 10.1016/j.cub.2007.01.026 ) and
more recently, the collared lemming
(doi: 10.1073/pnas.1213322109 ).
"We actually started the lemming
study about the same time as the
mammoth study, so the two projects
have influenced each other", said Dr
Dalén in email. Dr Dalén was a co-
author on the lemming paper, too.
"In some cases, it seems that the
population dynamics (local
extinctions and recolonisations) may
have been correlated between
mammoths and lemmings, which
would imply that changes in the
environment (driven by climate)
were behind these events".
"[O]ur recent paper on collared
lemmings seems to support
environmental factors as a driver of
population size for all of the arctic
specialist fauna", said co-author Ian
Barnes, a professor of biological
sciences and a reader in molecular
palaeobiology at the University of
London. Professor Barnes was a co-
author on both this paper and on the
lemming paper.
"[I]f you want to understand why
large mammals go extinct in the last
ice age, you need to understand what
small mammal populations are
doing", explained Professor Barnes in
email.
"In brief, during warm periods, an
increase in shrubs and drop in dry
steppe grassland makes life difficult
for arctic specialists, who tend to
retreat first into northern Asia, and
latterly into the High Arctic (and
finally into extinction, in the case of
the large mammals)."
But having survived several previous
warming periods, why did the woolly
mammoth go extinct after this most
recent warming period? Did disease
or hunting finally push them over the
edge?
"Thus far, no disease has been
identified that could possibly have
affected a massive suite of
taxonomically distinct large mammal
species, but not small mammals", said
Professor Barnes.
"As for humans causing the
extinction, our study doesn't provide
any evidence supporting this idea, but
on the other hand there is no data
disproving it either", said Dr Dalén in
email.
"But the mammoth didn't become
completely extinct at the end of the
last ice age, since it survived another
5,000 years on Wrangel Island",
continued Dr Dalén in email.
"What caused this population (and
consequently the species as a whole)
to go extinct is unknown. It could
have been climate, humans or
inbreeding (these are the three main
hypotheses)", said Dr Dalén. "Or even
disease".
Dr Dalén did remind me that if the
current warm period (the Holocene)
"hadn't been so darn long" -- more
than 10,000 years -- mammoths
likely would still be alive.
Like most good research, this study
raises more questions than it
answers.
"I really like the last point about the
future need to look at why mammoths
survived the previous warm periods,
but died out in the recent one", said
evolutionary biologist Tom Gilbert, a
professor at the University of
Copenhagen. Professor Gilbert was
not involved in this study.
"That's really to me the most
interesting question out there."
