Divided by DNA: The uneasy relationship between archaeology and ancient genomics
Two fields in the midst of a technological revolution are struggling to reconcile their views of the past.
The
West Kennet long barrow served as a tomb and ceremonial site for more
than a millennium. Credit: Robert Harvey/Natural World Photography.
Thirty kilometres north of Stonehenge, through
the rolling countryside of southwest England, stands a less-famous
window into Neolithic Britain. Established around 3600 bc
by early farming communities, the West Kennet long barrow is an earthen
mound with five chambers, adorned with giant stone slabs. At first, it
served as a tomb for some three dozen men, women and children. But
people continued to visit for more than 1,000 years, filling the
chambers with relics such as pottery and beads that have been
interpreted as tributes to ancestors or gods.
The artefacts offer a
view of those visitors and their relationship with the wider world.
Changes in pottery styles there sometimes echoed distant trends in
continental Europe, such as the appearance of bell-shaped beakers — a
connection that signals the arrival of new ideas and people in Britain.
But many archaeologists think these material shifts meshed into a
generally stable culture that continued to follow its traditions for
centuries.
“The ways in which people are doing things are the
same. They’re just using different material culture — different pots,”
says Neil Carlin at University College Dublin, who studies Ireland and
Britain’s transition from the Neolithic into the Copper and Bronze Ages.
But last year, reports started circulating that seemed to challenge this picture of stability. A study1
analysing genome-wide data from 170 ancient Europeans, including 100
associated with Bell Beaker-style artefacts, suggested that the people
who had built the barrow and buried their dead there had all but
vanished by 2000 bc. The genetic
ancestry of Neolithic Britons, according to the study, was almost
entirely displaced. Yet somehow the new arrivals carried on with many of
the Britons’ traditions. “That didn’t fit for me,” says Carlin, who has
been struggling to reconcile his research with the DNA findings.
The Bell Beaker ‘bombshell’ study appeared in Nature2
in February and included 230 more samples, to make it the largest
ancient-genome study on record. But it is just the latest example of the
disruptive influence that genetics has had on the study of the human
past. Since 2010, when the first ancient-human genome was fully
sequenced3, researchers have amassed data on more than 1,300 individuals (see ‘Ancient genomes’ graphic), and used them to chart the emergence of agriculture, the spread of languages and the disappearance of pottery styles — topics that archaeologists have laboured over for decades.
Some
archaeologists are ecstatic over the possibilities offered by the new
technology. Ancient-DNA work has breathed new life and excitement into
their work, and they are beginning once-inconceivable investigations,
such as sequencing the genome of every individual from a single
graveyard. But others are cautious.
“Half the archaeologists think
ancient DNA can solve everything. The other half think ancient DNA is
the devil’s work,” quips Philipp Stockhammer, a researcher at
Ludwig-Maximilians University in Munich, Germany, who works closely with
geneticists and molecular biologists at an institute in Germany that
was set up a few years ago to build bridges between the disciplines. The
technology is no silver bullet, he says, but archaeologists ignore it
at their peril.
Some archaeologists, however, worry that the
molecular approach has robbed the field of nuance. They are concerned by
sweeping DNA studies that they say make unwarranted, and even
dangerous, assumptions about links between biology and culture. “They
give the impression that they’ve sorted it out,” says Marc Vander
Linden, an archaeologist at the University of Cambridge, UK. “That’s a
little bit irritating.”
This isn’t the first time archaeologists
have had to contend with transformative technology. “The study of
prehistory today is in crisis,” wrote Cambridge archaeologist Colin
Renfrew in his 1973 book Before Civilization, describing the
impact of radiocarbon dating. Before the technique was developed by
chemists and physicists in the 1940s and 50s, prehistorians determined
the age of sites using ‘relative chronologies’, in some cases relying on
ancient Egyptian calendars and false assumptions about the spread of
ideas from the Near East. “Much of prehistory, as written in the
existing textbooks is inadequate: some of it, quite simply wrong,”
Renfrew surmised.
It wasn’t an easy changeover — early
carbon-dating efforts were off by hundreds of years or more — but the
technique eventually allowed archaeologists to stop spending most of
their time worrying about the age of bones and artefacts and focus
instead on what the remains meant, argues Kristian Kristiansen, who
studies the Bronze Age at the University of Gothenburg in Sweden.
“Suddenly there was a lot of free intellectual time to start thinking
about prehistoric societies and how they are organized.” Ancient DNA now
offers the same opportunity, says Kristiansen, who has become one of
his field’s biggest cheerleaders for the technology.
Genetics and archaeology have been uneasy bedfellows for more than 30 years — the first ancient-human DNA paper4,
in 1985, reported sequences from an Egyptian mummy (now thought to be
contamination). But improvements in sequencing technology in the
mid-to-late 2000s set the fields on a collision course.
In 2010,
scientists led by Eske Willerslev at the Natural History Museum of
Denmark used DNA from a lock of hair from a 4,000-year-old native
Greenlander to generate the first complete sequence of an ancient-human
genome3.
Seeing the future of the field before his eyes, Kristiansen asked
Willerslev to team up on a prestigious European Research Council grant
that would allow them to examine human mobility as the late Neolithic
gave way to the Bronze Age, some 4,000–5,000 years ago.
Association problems
Migration
has been a major source of tension for archaeologists. They have
debated at length whether human movements are responsible for cultural
changes in the archaeological record, such as the Bell Beaker
phenomenon, or whether it is simply the ideas that are moving through
cultural exchanges. Populations identified by the artefacts they
associated with came to be seen as a remnant of the science’s colonial
past, and one that imposed artificial categories. “Pots are pots, not
people,” goes a common refrain.
Most archaeologists have since cast aside the view that prehistory was like a game of Risk,
in which homogenous cultural groups conquer their way across a map of
the world. Instead, researchers tend to focus on understanding a small
number of ancient sites and the lives of the people who lived there.
“Archaeology had moved away from these grand narratives,” says Tom
Booth, a bioarchaeologist at the Natural History Museum in London, who
is part of a team using ancient DNA to trace the arrival of farming in
Britain. “A lot of people thought you needed to understand change
regionally to understand people’s lives.”
Ancient-DNA work — which
has repeatedly shown that a region’s modern inhabitants are often
distinct from populations that lived there in the past — promised, for
better or worse, to bring back some of the broad focus on migration to
human prehistory. “What genetics is particularly good at is detecting
change in populations,” says David Reich, a population geneticist at
Harvard Medical School in Boston, Massachusetts. Archaeologists,
Kristiansen says, “were prepared to accept that individuals had
travelled”. But for the Bronze Age period that he studies, “they were
not prepared for major migrations. That was a new thing.”
Studies of strontium isotopes in teeth5,
which vary with local geochemistry, had hinted that some Bronze Age
individuals had moved hundreds of kilometres over their lifetimes,
Kristiansen says. He and Willerslev wondered whether DNA analysis might
detect movements of whole populations during this period.
Bell Beaker pots signal a period of unprecedented cultural intermingling for early Europeans.Credit: Ashmolean Museum/Univ. Oxford/Bridgeman
They would have competition. In 2012, David Anthony, an
archaeologist at Hartwick College in Oneonta, New York, loaded his car
with boxes of human remains that he and his colleagues had excavated
from the steppes near the Russian city of Samara, including bones
associated with a Bronze Age pastoralist culture called the Yamnaya. He
was bringing them to the ancient-DNA lab just established by Reich in Boston. Like Kristiansen, Anthony was comfortable theorizing about the past on a grand scale. His 2007 book The Horse, the Wheel and Language proposed
that the Eurasian steppe had been a melting pot for the modern
developments of horse domestication and wheeled transport, which
propelled the spread of a family of languages called Indo-European
across Europe and parts of Asia.
In duelling 2015 Nature papers6,7, the teams arrived at broadly similar conclusions:
an influx of herders from the grassland steppes of present-day Russia
and Ukraine — linked to Yamnaya cultural artefacts and practices such as
pit burial mounds — had replaced much of the gene pool of central and
Western Europe around 4,500–5,000 years ago. This was coincident with
the disappearance of Neolithic pottery, burial styles and other cultural
expressions and the emergence of Corded Ware cultural artefacts, which
are distributed throughout northern and central Europe. “These results
were a shock to the archaeological community,” Kristiansen says. Cord cutters
The
conclusions immediately met with push-back. Some of it began even
before the papers were published, says Reich. When he circulated a draft
among his dozens of collaborators, several archaeologists quit the
project. To many, the idea that people linked to Corded Ware had
replaced Neolithic groups in Western Europe was eerily reminiscent of
the ideas of Gustaf Kossinna, the early-twentieth-century German
archaeologist who had connected Corded Ware culture to the people of
modern Germany and promoted a ‘Risk board’ view of prehistory known as settlement archaeology. The idea later fed into Nazi ideology.
Reich
won his co-authors back by explicitly rejecting Kossinna’s ideas in an
essay included in the paper’s 141-page supplementary material7.
He says the episode was eye-opening in showing how a wider audience
would perceive genetic studies claiming large-scale ancient migrations.
Still, not everyone was satisfied. In an essay8
titled ‘Kossinna’s Smile’, archaeologist Volker Heyd at the University
of Bristol, UK, disagreed, not with the conclusion that people moved
west from the steppe, but with how their genetic signatures were
conflated with complex cultural expressions. Corded Ware and Yamnaya
burials are more different than they are similar, and there is evidence
of cultural exchange, at least, between the Russian steppe and regions
west that predate Yamnaya culture, he says. None of these facts negates
the conclusions of the genetics papers, but they underscore the
insufficiency of the articles in addressing the questions that
archaeologists are interested in, he argued. “While I have no doubt they
are basically right, it is the complexity of the past that is not
reflected,” Heyd wrote, before issuing a call to arms. “Instead of
letting geneticists determine the agenda and set the message, we should
teach them about complexity in past human actions.”
Ann Horsburgh,
a molecular anthropologist and prehistorian at Southern Methodist
University in Dallas, Texas, attributes such tensions to communication
problems. Archaeology and genetics say distinct things about the past,
but often use similar terms, such as the name of a material culture.
“It’s C. P. Snow all over again,” she says, referring to the influential
‘Two Cultures’ lectures by the British scientist lamenting the deep
intellectual divide between the sciences and the humanities. Horsburgh
complains that genetic results are too often given precedence over
inferences about the past from archaeology and anthropology, and that
such “molecular chauvinism” prevents meaningful engagement9.
“It’s as though genetic data, because they’re generated by people in
lab coats, have some sort of unalloyed truth about the Universe.”
Horsburgh, who is seeing her own field of African prehistory start to feel the tremors of ancient genomics,
says that archaeologists frustrated at having their work misinterpreted
should wield their power over archaeological remains to demand more
equitable partnerships with geneticists. “Collaboration doesn’t mean I
send you an e-mail saying ‘hey, you’ve got some really cool bones. I’ll
get you a Nature paper.’ That’s not a collaboration,” she says.
Many
archaeologists are also trying to understand and engage with the
inconvenient findings from genetics. Carlin, for instance, says that the
Bell Beaker genome study sent him on “a journey of reflection” in which
he questioned his own views about the past. He has pored over the
selection of DNA samples included in the study as well as the basis for
its conclusion that the appearance of Bell Beaker artefacts coincided
with a greater than 90% replacement in Britain’s gene pool. “I didn’t
want to be questioning it from a position of ignorance,” Carlin says.
Like
Heyd, he accepts that a shift in ancestry occurred (although he has
questions about its timing and scale). Those results, in fact, now have
him wondering about how cultural practices such as leaving pottery and
other tributes at the West Kennet long barrow persisted in the face of
such upheavals. “I would characterize a lot of these papers as ‘map and
describe’. They’re looking at the movement of genetic signatures, but in
terms of how or why that’s happening, those things aren’t being
explored,” says Carlin, who is no longer disturbed by the disconnect. “I
am increasingly reconciling myself to the view that archaeology and
ancient DNA are telling different stories.” The changes in cultural and
social practices that he studies might coincide with the population
shifts that Reich and his team are uncovering, but they don’t
necessarily have to. And such biological insights will never fully
explain the human experiences captured in the archaeological record.
Reich
agrees that his field is in a “map-making phase”, and that genetics is
only sketching out the rough contours of the past. Sweeping conclusions,
such as those put forth in the 2015 steppe migration papers, will give
way to regionally focused studies with more subtlety.
This is
already starting to happen. Although the Bell Beaker study found a
profound shift in the genetic make-up of Britain, it rejected the notion
that the cultural phenomenon was associated with a single population.
In Iberia, individuals buried with Bell Beaker goods were closely
related to earlier local populations and shared little ancestry with
Beaker-associated individuals from northern Europe (who were related to
steppe groups such as the Yamnaya). The pots did the moving, not the
people.
Reich describes his role as that of a ‘midwife’ delivering
ancient-DNA technology to archaeologists, who can apply it as they see
fit. “Archaeologists will embrace this technology and will not be
Luddites,” he predicts, “and they’ll make it their own.” A stronger partnership
Nestled
in a sleepy valley in the state of Thuringia in former East Germany,
the city of Jena has become an unlikely hub for the convergence of
archaeology and genetics. In 2014, the prestigious Max Planck Society
established an Institute for the Science of Human History there and
installed a rising star in ancient-DNA research, Johannes Krause, as a
director. Krause was a protégé of the geneticist Svante Pääbo, at the
Max Planck Institute for Evolutionary Anthropology in Leipzig. There,
Krause worked on the Neanderthal genome10 and helped discover a new archaic human group, known as Denisovans11.
Whereas
Pääbo was focused on applying genetics to biological questions about
ancient humans and their relatives, Krause saw a wider scope for the
technology. Before leading the Jena institute, his team identified DNA
from plague-causing bacteria in the teeth of people who died from the
Black Death in the fourteenth century, the first direct evidence of a
potential cause for the pandemic12.
At Jena, Krause hoped to bring genetics to bear, not just on
‘prehistorical’ periods such as the Neolithic and the Bronze Age, where
archaeological methods are the main tool for reconstructing the past,
but also on more-recent times. Outreach with historians is still a work
in progress, but archaeology and genetics are thoroughly embedded at the
institute. The department Krause directs is even called
archaeogenetics. “We have to be interdisciplinary,” he says, because
geneticists are addressing questions and time periods that
archaeologists, linguists and historians have been poring over for
decades.
Krause and his team have been heavily involved in the
map-making phase of ancient genomics (he worked closely with Reich’s
team on many such projects). But a study published late last year13
that focused on the transition between the Neolithic and Bronze Age in
Germany won plaudits from archaeologists who have been dubious of the
larger-scale ancient-DNA studies.
Led by Stockhammer, who also has
a post at the Jena institute, the team analysed 84 Neolithic and Bronze
Age skeletons from southern Bavaria’s Lech River Valley dating to
between 2500 and 1700 bc. The diversity
in the genomes of cellular structures known as mitochondria, which are
inherited maternally, rose during this period, suggesting an influx of
women. Meanwhile, strontium isotope levels in teeth — which are set
during childhood — suggested that most females weren’t local. In one
case, two related individuals who lived within a few generations of each
other were found buried with different material cultures. In other
words, some cultural shifts in the archaeological record could be due
not to massive migrations, but to the systematic mobility of individual
women.
It is the prospect of more such studies that has
archaeologists salivating over ancient DNA. In the near future, says
Stockhammer, archaeologists will be able to sequence the genomes of all
the individuals at a burial site and build a local family tree, while
also determining how individuals fit into larger ancestry patterns. This
should allow researchers to ask how biological kinship relates to the
inheritance of material culture or status. “These are the big questions
of history. They can be solved now only with collaboration,” says
Stockhammer.
Another glimpse of this approach appeared in February on the bioRxiv preprint server14.
The paper explores Europe’s migration period, when ‘barbarian hordes’
filled the void left after the fall of the Roman Empire. In the paper, a
team of geneticists, archaeologists and historians built family trees
of 63 individuals from two medieval cemeteries in Hungary and northern
Italy associated with a group known as the Longobards. They found
evidence of high-status outsiders buried in the cemetery: most bore
central and northern European genetic ancestry that differed from that
of local people, who tended to be buried without goods — offering
tentative support to the idea that some barbarian groups included
outsiders.
Patrick Geary, a medieval historian at the Institute
for Advanced Study in Princeton, New Jersey, who co-led the Longobard
study, would not comment on the research because it is now being peer
reviewed. But he says that genetic studies of historical times, such as the migration period, carry pitfalls, too.
Historians are increasingly incorporating data such as palaeoclimate
records into their work, and will do likewise with ancient DNA, Geary
says. But they share archaeologists’ fears that biology and culture will
be conflated, and that problematic designations such as Franks or Goths
or Vikings will be reified by genetic profiles, overriding insights
into how ancient peoples viewed themselves. “These days, what historians
want to know about is identity,” he says. “Genetics cannot answer these
questions.”
Reich concedes that his field hasn’t always handled
the past with the nuance or accuracy that archaeologists and historians
would like. But he hopes they will eventually be swayed by the insights
his field can bring. “We’re barbarians coming late to the study of the
human past,” Reich says. “But it’s dangerous to ignore barbarians.”
Nature555, 573-576 (2018)
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