COMMENT GEOSCIENCE Did the GENETICS Dark start of ARCTIC Early-career climate- EARTH OBSERVATION AI experts Anthropocene heredity science traced to science collaborations needed to map road- begin in the 1600s? p.26 asylum statistics p.28 transcend tensions p.30 building boom p.30 ROMAN UCHYTEL An artist’s impression of the extinct woolly rhinoceros (Coelodonta antiquitatis). Big data little help in megafauna mysteries Too many meta-analyses of extinctions of giant kangaroos or huge sloths use data that are poor or poorly understood, warn Gilbert J. Price and colleagues. n March, the last male northern white Was the culprit early humans who Understanding why some groups rhinoceros died. The sub-species joins a dispersed from Africa more than 75,000 succumbed while others survived could pro- long list of large land animals that have years ago? Or was it climate change? The vide insights into how modern-day species Igone extinct over the past 100,000 years. latest way to try to settle the debate involves might — or might not — survive climatic The reason for the demise of the northern meta-analyses. These attempt to link the tim- and environmental changes, and into the white rhinoceros (Ceratotherium simum ing of extinctions to shifts in the climate, or to resilience of natural ecosystems to increas- cottoni) is undisputed: poaching and land evidence of the first appearance of humans in ing anthropogenic impact. disturbance by people. By contrast, who or a particular region. Over the past five years, But in our view, the ‘big-data’ approach what caused the extinctions of mammoths, the number of meta-analyses has greatly cannot, at this point, get us closer to an enormous ground sloths and other Quater- increased (see ‘In fashion’). Many have been answer. There simply aren’t enough good- nary megafauna remains one of the most con- published in high-impact journals, and they quality data. An understanding of what drove tested topics in the historical sciences. are starting to shape the debate. the extinctions requires detailed analysis ©2018 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts rese7rv JUNEed. 2018 | VOL 558 | NATURE | 23 COMMENT on a species-by-species basis. This means investing effort into finding more fossil speci- mens and verifying the ages of those that have already been discovered using improved dating methods. It also means relating the ROMAN UCHYTEL timing of species’ existence and disappear- ance to detailed local environmental, climatic and archaeological records. HUMAN LINK For a typical meta-analysis, researchers mine the literature for dates associated with now- extinct megafauna, as well as for estimates of when humans arrived at a particular region (on the basis of archaeological and other data). In some cases, they then combine these records with global-scale palaeocli- mate data, such as those obtained from ice cores collected from the Arctic. By mapping correlations between events, investigators try to identify the dominant factor driving species losses. Over the past two decades, most of the meta-analyses that merge continental or Artist’s impression of the extinct land crocodile, a giant kangaroo and a giant wombat-like marsupial. global-scale data sets have pointed the finger at modern humans. In fact, some researchers that have never even been dated, directly or the pollen and spore fossil record. Because contend that the results are so clear that there indirectly. A 2016 study4, for instance, listed it occurs on animal dung, an abundance of is no need for further debate1. Australian animals such as the land crocodile it in a sediment core is often taken to indi- For any meta-analysis, however, the Quinkana and the giant wombat Ramsayia cate high numbers of big herbivores. Some reliability of results is largely governed by the among the megafauna thought to have investigators assume that a decrease in the ‘GIGO’ principle: garbage in, garbage out. In existed in the past 100,000 years. The fos- appearance of the fungus over time and our view, most of these analyses depend on sils of these species have never been dated9. its eventual disappearance from the fossil questionable data, making the results hard (More than 25 of Australia’s megafaunal spe- record signal the extinction of megafauna11. to interpret at best. Six key problems under- cies, or around 30%, have never been dated, Yet Sporormiella lives on the excrement of mine many of the studies conducted so far. simply because no one has done the work.) a vast range of both big and small animals, including mammals and birds, herbivores and Outdated geochronological information. Insufficient data. Some meta-analyses take even some carnivores12. Its abundance is also Models frequently use data from studies that the last appearance of a species in the fossil affected by factors such as climate and water have been super- record to be the time when the animal went flow. Thus, on its own, levels of Sporormiella seded. For instance, “With good data, extinct5. In the rare cases where hundreds in a pollen core can’t provide information during the 1980s, models could of samples have been found, for instance about which species were present at any one radiocarbon dat- provide crucial for mammoths and mastodons, a species’ time, or in what numbers. ing of species such insights about disappearance from the fossil record could as the Eurasian large-scale well signal its demise. Yet where only a few Insufficient scrutiny. Lastly, long lists of woolly rhinoceros changes.” specimens exist, the last appearance in the extinct species (frequently just names and (Coelodonta antiq- fossil record might have little bearing on the numbers in supplementary materials) uitatis) suggested that it survived well into timing of the extinction. often do not receive the necessary level of the Holocene — perhaps until as recently A step in the right direction are probabilis- scrutiny. This has led to some unfortunate as 3,600 years ago2. But refinements in dat- tic models of extinction times. These incor- errors. The authors of at least two studies4,13 ing methods have shown that the rhinos had porate a degree of error associated with the have argued, for instance, that Homo sapiens actually disappeared by about 14,000 years age of specimens, based in part on the qual- caused the demise of giant marsupials such ago3. Some of the most recent big-data stud- ity of the methods used to date them. Again, as Euryzygoma dunense and Euowenia grata. ies still use erroneous early dates for the rhino4 the robustness of the results depends on the These were extinct for millions of years before and other species5. quality of the data fed in. At this point, very Homo sapiens even appeared; they are known few of the species that went extinct over the only from the Pliocene, the period 5.3 million Contested dates. In other cases, the dates past 100,000 years are associated with reliable to 2.6 million years ago. Another paper14 sug- associated with certain species are still in dates10. (In our view, the cave lion (Panthera gested that the genus Macropus went extinct question. For instance, researchers first esti- spelaea), woolly rhino and woolly mammoth in Australia some 40,000 years ago. In fact, mated the age of the elephant-like Stegodon (Mammuthus primigenius) are among the Macropus is alive and kicking: it includes Aus- trigonocephalus not by dating the fossils handful of species for which sufficient data tralia’s extant kangaroos. themselves, but by dating fossils from other exist to enable a modelling approach.) animals collected from deposits more than Arbitrary definition. Megafauna are 100 kilometres away6. Other investigators Problematic proxies. In the absence of fossil commonly defined as Quaternary ter- have flagged problems associated with using bones, some researchers have used proxy restrial vertebrates with a mass of at least inferred ages7, yet these continue to be fed data to test megafaunal extinction hypo- 44 kilograms — roughly 100 pounds. This into meta-analyses8. theses. For instance, the coprophilous fun- is a nice, round cut-off, but it is essentially In some cases, ages are assigned to species gus Sporormiella is a common component of arbitrary. Also, in some cases, ‘megafauna’ 24 | NATURE | VOL 558 | 7 JUNE 2018©2018 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. ©2018 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. COMMENT are not so mega. For instance, they could IN FASHION of years ago. This is thanks to various include extinct terrestrial vertebrates that Meta-analyses are increasingly being used to study advances, such as combined U-series and are larger than their extant cousins but that the drivers of past extinctions of big animals. electron-spin resonance dating. Other weigh considerably less than 44 kilograms. emerging techniques, such as the extrac- SOURCE: G. PRICE An extinct relative of the modern-day Aus- = 1 paper tion and analysis of ancient DNA, can shed tralian echidna — Megalibgwilia ramsayi light on changes to the population size of — is considered to be megafauna, even now-extinct species. Several studies have though it weighed only around 15 kilo- used such approaches to demonstrate that grams when it existed (until at least around 1997 2002 2007 2012 2017 populations of taxa, from giant Irish elk 100,000 years ago). (Megaloceros giganteus)17 to the Beringian *Studies included are those that have modelled large data sets of 18 In other words, megafauna are highly published ages. steppe bison (Bison priscus) , plummeted biologically and ecologically diverse, with many thousands of years before their ulti- several species separated from each other by or in the deposition of sand dunes, might mate extinction, apparently because of dete- hundreds of millions of years of evolution.