NEWS | IN DEPTH Ceres’s gravity field, the Dawn team will at- thrusters that rely on a limited 45-kilogram ANCIENT DNA tempt to sort out the dimensions of these lay- supply of hydrazine. ers—and figure out whether any liquid water Rayman says that the team has hoarded remains in the ice layer. The Dawn team will the hydrazine carefully. On the trip from Indo-European also try to determine whether the hundreds Vesta to Ceres, the spacecraft spun to aim of meters of dirt blanketing the surface are its antenna toward Earth for a “check-in” debris from impacts, the leftovers after dirty once every 4 weeks rather than once a week languages tied ice has sublimated away, or rocky material as planned. Once in orbit around Ceres, somehow exhumed from deep underground. Dawn will spin to send data back to Earth That third possibility gained support in less often than planned, storing more data to herders 2006, when Rivkin and his colleagues, us- on board. As a result, the main mission will Ancient migration from ing ground-based telescopes, saw evidence take longer and won’t be completed until of iron-rich clays and carbonates at the sur- June 2016. But Rayman is proud that Dawn the east shaped European face. These minerals often form when hot won’t have to compromise mission goals. He water encounters rock—the conditions that expects to finish with 19 of the 28 kilograms genome and language may once have prevailed at the base of the of hydrazine unused. ocean, where it met a hot, rocky core, Rivkin Rivkin hopes that what Dawn finds will By Michael Balter and Ann Gibbons says. Frigid cryovolcanoes perched atop fis- ensure that this won’t be the last robotic sures in the ice could have spewed the min- visitor to Ceres. “We’re only now getting to espite their allegiances to 47 different erals onto the surface. Now Dawn scientists the point where the pieces are starting to nations, 87 ethnic groups, and count- hope to map those minerals to learn when fall into place,” he says. “It’d be a great place less football teams, Europeans have those volcanoes erupted and how wide- to put a rover.” ■ a lot in common. Most speak closely spread they were. related languages that are members Last year came a hint that Ceres’s ocean of the great Indo-European language may still be liquid. Scientists using Her- Dfamily. Now a new study uses ancient DNA schel, an infrared space telescope, reported Iced over to suggest that a massive migration of herd- finding faint traces of water vapor escaping Under an icy mantle, Ceres may harbor a thin ers from the east shaped the genomes of on November 8, 2015 from the surface. Few scientists expect to ocean of liquid water, some of which could most living Europeans—and that these find geysers like the ones that spew from spew to the surface in cryovolcanoes. immigrants may have been the source of Enceladus—but the wispy plumes could Proto-Indo-European (PIE), the mysterious indicate that water is being squeezed from ancestral tongue from which the more than subterranean pockets of melt. On the other 400 Indo-European languages sprang. hand, Raymond says, they could be due to Based on DNA gathered from dozens of the sublimation of ice exposed by a recent ancient skeletons across Europe and Asia, asteroid impact. the study, described last week in a preprint A thrifty ion propulsion system powered posted on the bioRxiv server but not yet www.sciencemag.org Dawn’s journey to Ceres. The system, which published in a journal, reveals when and uses electricity to charge and expel small where different groups of people arrived in amounts of xenon, delivers a push so gentle Europe and interbred with each other. One that it would take 9 days for the spacecraft surprise is that a migration of herders from to accelerate from 0 to 100 kilometers per the steppes of today’s Russia and Ukraine hour, Rayman says. “It’s what I like to call about 4500 years ago significantly shifted acceleration with patience.” But the ion en- the genetic makeup of today’s Europeans. Downloaded from gine uses a tenth as much propellant as a “What we know now as ‘the European ge- chemical rocket. That allowed engineers to nome’ didn’t come into existence until the keep the thrusters on for nearly two-thirds Bronze Age,” says evolutionary biologist of the mission and made Dawn’s complex Greger Larson of the University of Oxford 950 km trajectory possible. in the United Kingdom. “Lots of people Another technical component has caused hung out in Europe before then, but their major headaches: the spacecraft’s reac- Thin dirty crust Liquid ocean genetic makeup didn’t closely mirror mod- tion wheels, spinning disks that control Ice mantle Hydrated silicates ern Europeans.” where the spacecraft points along three Almost everyone praises the new genetic axes. Just before launch, Rayman says, the Rocky core data, which include nuclear DNA from twice team was warned that wheels of this type as many ancient Europeans as all previous had a checkered history, but it was too Cryovolcano analyses combined. But the researchers go late to replace them. (Failures of the same further, suggesting that these herders, the type of wheels later crippled the Kepler Yamnaya people, spoke either PIE or an exoplanet-hunting spacecraft.) Dawn’s first early form of Indo-European language and SCIENCE reaction wheel conked out in June 2010, brought it to central Europe. Some critics and a second failed in August 2012, as the say that connecting populations identified by Ice spacecraft was spiraling away from Vesta. DNA to any specific language or culture goes Mission managers turned off the remaining too far. The location of the so-called Indo- two wheels and shifted to a backup plan, in Liquid ocean European homeland where PIE was first which they would point the spacecraft with spoken, “is not nailed down yet,” says ancient GRULLÓN/ G. ILLUSTRATION: 814 20 FEBRUARY 2015 • VOL 347 ISSUE 6224 sciencemag.org SCIENCE Published by AAAS DNA specialist Pontus Skoglund. analyzed all Indo-European languages us- Up until the 1980s, linguists and archae- ing methods borrowed from biology for ologists embraced variations of the steppe crafting evolutionary trees. They concluded hypothesis for the origin of PIE. Then in that the origins of PIE date to about 6000 1987, archaeologist Colin Renfrew of the years ago, consistent with the steppe hypoth- University of Cambridge in the United King- esis but not the earlier Anatolian one (see dom proposed that early farmers of Anatolia http://scim.ag/ProtoIndoEuro). (present-day Turkey) and the Middle East But some researchers remain unconvinced spoke this ancestral tongue and brought it that PIE came from the steppes. Linguist with them as they expanded both east and Paul Heggarty of the Max Planck Institute west about 8000 years ago. Signs of this mass for Evolutionary Anthropology in Leipzig, migration of farmers out of the Middle East Germany, criticizes the linguistic analysis for show up in the genomes of living and ancient The people who made this ancient Corded Ware pot predetermining the relatedness of certain people, boosting the so-called Anatolian hy- in central Europe may have descended from herders. languages, which could bias the result. pothesis. But most of this evidence comes The links between genes and language are from only a few individuals or from mater- according to the paper. That suggests a mas- still weak, Larson adds. “This data is super nally inherited mtDNA, making it difficult to sive migration of Yamnaya people from their solid and phenomenally revelatory when know whether the findings represented true steppe homeland into eastern Europe about thinking about the origin of the modern population movements. 4500 years ago when the Corded Ware cul- European genome,” he says. “But the whole A group led by geneticist David Reich of ture began, perhaps carrying an early form Indo-European thing is interesting, but a Harvard Medical School in Boston recently of Indo-European language. speculative leap away from the raw data.” developed a method to get a genetic finger- The Corded Ware culture then spread rap- Meanwhile, Renfrew is holding steady to print from ancient skeletons without the idly across north and central Europe, carry- his Anatolian hypothesis. The movement time and expense of sequencing their entire ing the Yamnaya genetic signature as far as out of the steppes seen in the ancient DNA genome. His team has now analyzed DNA today’s Scandinavia. The steppe ancestry is data, he says, “may be a secondary migra- from 69 Europeans who lived 3000 to 8000 “ubiquitous” in most living Europeans, the tion into central Europe, 3000 to 4000 years years ago, reading bases at 400,000 positions authors write. later than the spread of farmers which first across the genome that tend to vary among The results are a “smoking gun” for an an- brought Indo-European speech to Europe.” individuals. The DNA revealed patterns of cient migration into Europe from the steppes, In that case, the Yamnaya people may have ancestry, and the researchers also knew the and boost the odds that PIE emerged there, spoken not PIE, but an already derived Indo- culture associated with each skeleton, plus says Skoglund, who works in Reich’s lab but European tongue ancestral to today’s Balto- when and where each individual lived. So was not a co-author. “The results level the Slavic languages such as Russian and Polish.