<<

COMMENT GEOSCIENCE Did the GENETICS Dark start of 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 ( antiquitatis). Big data little help in mysteries Too many meta-analyses of of giant kangaroos or huge 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 who Understanding why some groups rhinoceros died. The sub-species joins a dispersed from more than 75,000 succumbed while others survived could pro- long list of large land that have ago? Or was it ? 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 (Ceratotherium simum ing of extinctions to shifts in the climate, or to resilience of natural ecosystems to increas- cottoni) is undisputed: 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 , 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 . 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. 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 , a giant kangaroo and a giant wombat-like . 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 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 . 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 ’s megafaunal spe- record signal the 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 and , 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 , 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 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 — 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 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 , 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 ( gested that the genus Macropus went extinct question. For instance, researchers first esti- spelaea), woolly rhino and woolly in Australia some 40,000 years ago. In fact, mated the age of the -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 , 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 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 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. hint at local landscape changes. And so on. riorating local climates and habitat changes. Researchers should not therefore expect them Broad global palaeo-temperature records Some might counter that we’re averse to to have responded in the same way to changes are likely to be a crude guide to climatic and change and are simply finding another reason in their environments — whether driven by environmental changes at local scales. to be alarmed about the demise of the field humans or by climate. For each species, investigators should also sciences in a digital world19. But our argument strive to develop a clearer understanding of is not with modelling per se. With good data, A BETTER WAY the human populations that lived alongside, models could provide crucial insights into We think that as long as data from the fossil and the nature of their interactions. This large-scale changes and the broad nature of record remain scant, an understanding could be obtained by analysing DNA sam- the interactions between humans and other of what drove the extinctions of large ani- ples extracted from ancient human remains, big animals as humans dispersed from Africa. mals over the past 100,000 years requires for instance, or by studying middens, ancient More data, of better quality, can be obtained detailed analysis on a species-by-species dumps for domestic waste. only through fieldwork and rigorous analysis basis. This means trying to find new fossils For example a study published earlier this of fossil materials. ■ and verifying the estimated age of specimens combined new dating approaches with previously found — for instance, through chemical analyses of the bones of the cave Gilbert J. Price is a senior lecturer repeated sampling, or by using improved (Ursus spelaeus), to show that its herbivo- in palaeontology at The University of techniques to date museum specimens. rous diet had remained unchanged up until its , Australia. Julien Louys is an It also means taking into account all the last appearance in , some 23,500 years Australian Research Council Future Fellow local palaeoenvironmental information that ago15. Moreover, cut marks on its bones have at Griffith University, Queensland, Australia. is available to develop a detailed understand- revealed that some of these animals were J. Tyler Faith is a curator of archaeology and ing of the palaeoecology of each species and hunted by humans. And researchers have an assistant professor of anthropology at the its ecosystem. To reconstruct the diet of an linked the of the extinct eastern University of Utah in Salt Lake City. Eline animal, researchers can use stable isotope African Damaliscus hypsodon to the Lorenzen is a curator and associate professor analyses of enamel. Pollen cores can open, dry grasslands it inhabited, to track the at the Natural History Museum of Denmark, indicate the local vegetation at the time. The demise of both16. København. Michael C. Westaway is a geochemistry of certain formations nearby, Megafaunal fossils can now be dated senior research fellow at Griffith University. such as stalagmites, might give clues about with much greater efficiency and preci- e-mail: [email protected] the local climate. Changes in the nature of the sion — including those of animals that sediment laid down in a nearby creek bed, existed several hundreds of thousands 1. Flannery, T. Q. Essay 48, 1–80 (2012). 2. Turvey, S. T., Tong, H., Stuart, A. J. & Lister, A. M. Quat. Sci. Rev. 76, 156–166 (2013) 3. Stuart, A. J. & Lister, A. M. Quat. Sci. Rev. 51, 1–17 (2012). 4. Bartlett, L. J. et al. Ecography 39, 152–161 (2016). 5. Wan, X. & Zhang, Z. Proc. R. Soc. B 284, 20162438 (2017). ROMAN UCHYTEL 6. van den Bergh, G. D., de Vos, J. & Sondaar, P. Y. Palaeogeogr. Palaeoclimatol. Palaeoecol. 171, 385–408 (2001). 7. Westaway, M. C. & Groves, C. P. Archaeol. Oceania 44, 84–95 (2009). 8. Sandom, C., Faurby, S., Sandel, B. & Svenning, J.-C. Proc. R. Soc. B 281, 20133254 (2014). 9. Wroe, S. et al. Proc. Natl Acad. Sci. 110, 8777–8781 (2013). 10. Stuart, A. J. Geol. J. 50, 338–363 (2015). 11. Gill, J. L., Williams, J. W., Jackson, S. T., Lininger, K. B. & Robinson, G. S. Science 326, 1100–1103 (2009). 12. Ahmed, S. I. & Cain, R. F. Can. J. Bot. 50, 419–477 (1972). 13. Johnson, C. N. Proc. R. Soc. B: Biol. Sci. 269, 2221–2227 (2002). 14. Saltre, F. et al. Nature Commun. 7, 10511 (2016). 15. Terlato, G. et al. Histor. Biol. https://doi.org/10.108 0/08912963.2018.1448395 (2018). 16. Faith, J. T. et al. Palaeogeogr. Palaeoclimatol. Palaeoecol. 361–362, 84–93 (2012). 17. Cooper, A. et al. Science 349, 602–606 (2015). 18. Shapiro, B. et al. Science 306, 1561–1565 (2004). 19. Ríos-Saldaña, C. A., Delibes-Mateos, M. & Ferreira, The giant wombat-like Euryzygoma went extinct long before Homo sapiens even evolved. C. C. Glob. Ecol. Conserv. 14, e00389 (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 rese7rv JUNEed. 2018 | VOL 558 | NATURE | 25