O mistério extinto da dieta do urso da caverna do vegetariano desvendou

por Taylor & Francis

 

 Crânio e mandíbula completos de um urso Deninger de Sima de los Huesos, na Espanha. Crédito: Javier Trueba (Madrid Scientific Films).

 

Durante o período tardio do Pleistoceno (entre 125.000 a 12.000 anos atrás), duas espécies de ursos percorreram a Europa: ursos pardos onívoros ( Ursus arctos ) e o extinto urso de caverna vegetariano ( Ursus spelaeus ).

 

Até agora, muito pouco se sabe sobre a evolução alimentar do urso da caverna e como ele se tornou vegetariano, pois os fósseis do ancestral direto, o urso Deninger ( Ursus deningeri ), são extremamente escassos.

 

No entanto, um artigo publicado na revista Historical Biology lança uma nova luz sobre isso. Uma equipe de pesquisa da Alemanha e da Espanha descobriu que o urso de Deninger provavelmente tinha uma dieta semelhante à de seu descendente - o urso-caverna clássico -, pois novas análises mostram uma morfologia distinta no crânio, mandíbula e dentes, o que tem sido relacionado à sua especialização na dieta de um animal. maior consumo de matéria vegetal.

 

Para entender a evolução da linhagem de ursos das cavernas, os pesquisadores examinaram os fósseis raros e removeram os sedimentos digitalmente, para não correr o risco de danificar os fósseis. Usando métodos estatísticos sofisticados, chamados morfometria geométrica, os pesquisadores compararam a forma tridimensional das mandíbulas e do crânio do urso de Deninger com a dos ursos das cavernas clássicos e modernos.

 

"As análises mostraram que o urso de Deninger tinha mandíbulas e crânio de forma muito semelhante ao clássico urso das cavernas", explica Anneke van Heteren, principal autora do estudo e chefe da seção de mamíferos da Coleção de Zoologia do Estado da Baviera. Isso implica que eles foram adaptados aos mesmos tipos de alimentos e eram principalmente vegetarianos.

 

 

Reconstruções por micro-TC de A) um crânio masculino subadulto do urso Deninger da Península Ibérica em diferentes pontos de vista em comparação com B) um crânio masculino adulto de um urso de caverna clássico. Os crânios são semelhantes em muitos aspectos, mas o crânio do urso da caverna é maior e mais robusto. Crédito: Elena Santos (Centro Mixto UCM-ISCIII) / Taylor e Francis.

"Existe uma discussão em andamento sobre até que ponto o urso-caverna clássico era vegetariano. E é por isso que as novas informações sobre a dieta de seu ancestral direto são tão importantes, porque nos ensinam que uma diferenciação entre a dieta de ursos de caverna e ursos pardos já foram estabelecidos há 500 mil anos e provavelmente mais cedo ", diz Mikel Arlegi, doutorado nas Universidades do País Basco e Bordeaux e co-autor do estudo.

 

Curiosamente, os pesquisadores também descobriram que há diferenças de forma entre os ursos de Deninger da Península Ibérica e os do resto da Europa, que provavelmente não estão relacionados à dieta .

 

Eles apresentaram três possibilidades para explicar essas diferenças: 

1) os ursos ibéricos são cronologicamente mais jovens que os demais; 

2) os Pirineus, atuando como barreira natural, resultaram em alguma diferenciação genética entre os ursos ibéricos e os do resto da Europa , 

3) havia várias linhagens, com apenas uma levando ao urso clássico da caverna ou cada linhagem levando a um grupo diferente de ursos da caverna.

 

"No entanto, são necessários mais fósseis para testar essas três hipóteses", disse Asier Gómez-Olivencia, pesquisador Ikerbasque da Universidade do País Basco.

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Antigo DNA de urso caverna extinto encontrado em ursos modernos

por Bob Yirka, Phys.org

Crédito: CC0 Public Domain

Uma equipe internacional de pesquisadores encontrou evidências de DNA extinto de ursos-da-caverna em ursos modernos. 

 Em seu artigo publicado na revista Nature Ecology & Evolution , o grupo descreve sua análise genética dos ursos marrons e polares modernos e como eles se comparam aos ursos de caverna extintos.

 

Os ursos das cavernas eram um tipo de urso que vivia na Ásia e na Europa. Eles eram um pouco maiores que os ursos marrons de hoje, mas alimentavam-se de vegetação em vez de carne. Eles foram extintos há aproximadamente 24.000 anos (durante o Último Máximo Glacial) por razões desconhecidas.  

Nesse novo esforço, a equipe de pesquisa concentrou-se em aprender por que os ursos foram extintos. Para esse fim, eles realizaram uma análise de DNA em amostras colhidas em quatro restos de urso de aproximadamente 35.000 anos atrás. Seu objetivo era estudar como a população de ursos das cavernas mudou de tamanho por longos períodos. Como um aparte, eles decidiram comparar o DNA dos ursos das cavernas extintas com os ursos modernos.

 

Os pesquisadores relatam que sua comparação revelou que os ursos marrons e polares modernos têm DNA de urso de caverna - em média 0,9 a 2,4% de seu genoma é DNA de urso de caverna. Eles observaram que os dois tipos de ursos modernos são mais geneticamente semelhantes do que os ursos das cavernas. 

Mas eles descobriram que qualquer um deles era capaz de acasalar-se com um urso da caverna e que seus filhotes também podiam se reproduzir com qualquer um dos três tipos. Eles apontam que seus resultados não são surpreendentes - as semelhanças entre os ursos, o período de sobreposição em que viviam e a proximidade de seu habitat tornaram a criação de animais quase uma certeza. A descoberta de DNA extinto de urso de caverna em ursos modernos é o primeiro exemplo de DNA de uma espécie extinta antiga encontrada em uma espécie moderna que não seja humana - temos restos de DNA neandertal em nossos genomas.

 

Cave caveira de urso. Os ursos das cavernas eram gigantes, muito maiores do que os ursos pardos vivos Crédito: Andrei Posmoșanu

Os pesquisadores sugerem que pesquisas futuras possam explorar o que significa uma espécie se extinguir - se seu DNA sobreviver em outras espécies, elas realmente desapareceram? Eles observam que seu estudo reafirma o fato de que espécies animais se cruzam, algumas regularmente. Esse poderia ter sido o caso dos ursos modernos e dos ursos das cavernas - os ursos das cavernas antigas também tinham DNA de urso moderno em seus pools genéticos.

 

Ossos de urso de caverna da era do gelo encontrados em sua antiga caverna de hibernação Crédito: Marius Robu

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O mistério extinto da dieta do urso da caverna do vegetariano desvendou

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Mais informações: Axel Barlow et al. Sobrevivência genômica parcial de ursos de caverna em ursos pardos vivos, Nature Ecology & Evolution (2018). DOI: 10.1038 / s41559-018-0654-8

  Resumo
 
Embora muitas espécies grandes de mamíferos tenham sido extintas no final da época do Pleistoceno, seu DNA pode persistir devido a episódios passados ​​de mistura interespécies. No entanto, evidências empíricas diretas da persistência de alelos antigos ainda são escassas. Aqui, apresentamos dados genômicos de cobertura múltipla de quatro ursos de caverna do Pleistoceno Superior (complexo Ursus spelaeus) e mostramos que os ursos de caverna hibridizaram com ursos marrons (Ursus arctos) durante o Pleistoceno. Desenvolvemos uma abordagem para avaliar a direcionalidade e o tempo relativo do fluxo gênico. Descobrimos que segmentos de DNA de ursos caverna ainda persistem nos genomas de ursos pardos vivos, com os ursos contribuindo de 0,9 a 2,4% dos genomas de todos os ursos pardos investigados. Nossos resultados mostram que, embora a extinção seja normalmente considerada absoluta, após a mistura, fragmentos do pool genético de espécies extintas podem sobreviver por dezenas de milhares de anos nos genomas das espécies receptoras existentes.

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.

Gilbert J. Price,

Julien Louys,

J. Tyler Faith,

Eline Lorenzen &

Michael C. Westaway

PDF version

An artist’s impression of the extinct woolly rhinoceros (Coelodonta antiquitatis).Credit: Roman Uchytel

In March, the last male northern white rhinoceros died. The sub-species joins a long list of large land animals that have gone extinct over the past 100,000 years.
The reason for the demise of the northern white rhinoceros (Ceratotherium simum cottoni) is undisputed: poaching and land disturbance by people. By contrast, who or what caused the extinctions of mammoths, enormous ground sloths and other Quaternary megafauna remains one of the most contested topics in the historical sciences.
Was the culprit early humans who dispersed from Africa more than 75,000 years ago? Or was it climate change? The latest way to try to settle the debate involves meta-analyses. These attempt to link the timing of extinctions to shifts in the climate, or to evidence of the first appearance of humans in a particular region. Over the past five years, the number of meta-analyses has greatly increased (see ‘In fashion’). Many have been published in high-impact journals, and they are starting to shape the debate.

Source: G. Price

Understanding why some groups succumbed while others survived could provide insights into how modern-day species might — or might not — survive climatic and environmental changes, and into the resilience of natural ecosystems to increasing anthropogenic impact.
But in our view, the ‘big-data’ approach cannot, at this point, get us closer to an answer. There simply aren’t enough good-quality data. An understanding of what drove the extinctions requires detailed analysis on a species-by-species basis. This means investing effort into finding more fossil specimens and verifying the ages of those that have already been discovered using improved dating methods. It also means relating the timing of species’ existence and disappearance 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 palaeoclimate 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 global-scale data sets have pointed the finger at modern humans. In fact, some researchers contend that the results are so clear that there is no need for further debate¹.
For any meta-analysis, however, the reliability of results is largely governed by the ‘GIGO’ principle: garbage in, garbage out. In our view, most of these analyses depend on questionable data, making the results hard to interpret at best. Six key problems undermine many of the studies conducted so far.
Outdated geochronological information. Models frequently use data from studies that have been superseded. For instance, during the 1980s, radiocarbon dating of species such as the Eurasian woolly rhinoceros (Coelodonta antiquitatis) suggested that it survived well into the Holocene — perhaps until as recently as 3,600 years ago². But refinements in dating methods have shown that the rhinos had actually disappeared by about 14,000 years ago³. Some of the most recent big-data studies still use erroneous early dates for the rhino⁴ and other species⁵.
Contested dates. In other cases, the dates associated with certain species are still in question. For instance, researchers first estimated the age of the elephant-like Stegodon trigonocephalus not by dating the fossils themselves, but by dating fossils from other animals collected from deposits more than 100 kilometres away⁶. Other investigators have flagged problems associated with using inferred ages⁷, yet these continue to be fed into meta-analyses⁸.

Artist’s impression of the extinct land crocodile, a giant kangaroo and a giant wombat-like marsupial.Credit: Roman Uchytel

In some cases, ages are assigned to species that have never even been dated, directly or indirectly. A 2016 study⁴, for instance, listed Australian animals such as the land crocodile Quinkana and the giant wombat Ramsayia among the megafauna thought to have existed in the past 100,000 years. The fossils of these species have never been dated⁹. (More than 25 of Australia’s megafaunal species, or around 30%, have never been dated, simply because no one has done the work.)

Insufficient data. Some meta-analyses take the last appearance of a species in the fossil record to be the time when the animal went extinct⁵. In the rare cases where hundreds of samples have been found, for instance for mammoths and mastodons, a species’ disappearance from the fossil record could well signal its demise. Yet where only a few specimens exist, the last appearance in the fossil record might have little bearing on the timing of the extinction.

A step in the right direction are probabilistic models of extinction times. These incorporate a degree of error associated with the age of specimens, based in part on the quality of the methods used to date them. Again, the robustness of the results depends on the quality of the data fed in. At this point, very few of the species that went extinct over the past 100,000 years are associated with reliable dates¹⁰. (In our view, the cave lion (Panthera spelaea), woolly rhino and woolly mammoth (Mammuthus primigenius) are among the handful of species for which sufficient data exist to enable a modelling approach.)

Problematic proxies. In the absence of fossil bones, some researchers have used proxy data to test megafaunal extinction hypotheses. For instance, the coprophilous fungus Sporormiella is a common component of the pollen and spore fossil record. Because it occurs on animal dung, an abundance of it in a sediment core is often taken to indicate high numbers of big herbivores. Some investigators assume that a decrease in the appearance of the fungus over time and its eventual disappearance from the fossil record signal the extinction of megafauna¹¹.

Yet Sporormiella lives on the excrement of a vast range of both big and small animals, including mammals and birds, herbivores and even some carnivores¹². Its abundance is also affected by factors such as climate and water flow. Thus, on its own, levels of Sporormiella in a pollen core can’t provide information about which species were present at any one time, or in what numbers.

Insufficient scrutiny. Lastly, long lists of extinct species (frequently just names and numbers in supplementary materials) often do not receive the necessary level of scrutiny. This has led to some unfortunate errors. The authors of at least two studies⁴,¹³ have argued, for instance, that Homo sapiens caused the demise of giant marsupials such as Euryzygoma dunense and Euowenia grata. These were extinct for millions of years before Homo sapiens even appeared; they are known only from the Pliocene, the period 5.3 million to 2.6 million years ago. Another paper¹⁴ suggested that the genus Macropus went extinct in Australia some 40,000 years ago. In fact, Macropus is alive and kicking: it includes Australia’s extant kangaroos.

Arbitrary definition. Megafauna are commonly defined as Quaternary terrestrial vertebrates with a mass of at least 44 kilograms — roughly 100 pounds. This is a nice, round cut-off, but it is essentially arbitrary. Also, in some cases, ‘megafauna’ are not so mega. For instance, they could include extinct terrestrial vertebrates that are larger than their extant cousins but that weigh considerably less than 44 kilograms. An extinct relative of the modern-day Australian echidna — Megalibgwilia ramsayi — is considered to be megafauna, even though it weighed only around 15 kilograms when it existed (until at least around 100,000 years ago).

In other words, megafauna are highly biologically and ecologically diverse, with several species separated from each other by hundreds of millions of years of evolution. Researchers should not therefore expect them to have responded in the same way to changes in their environments — whether driven by humans or by climate.

The giant wombat-like Euryzygoma went extinct long before Homo sapiens even evolved.Credit: Roman Uchytel

A better way
We think that as long as data from the fossil record remain scant, an understanding of what drove the extinctions of large animals over the past 100,000 years requires detailed analysis on a species-by-species basis. This means trying to find new fossils and verifying the estimated age of specimens previously found — for instance, through repeated sampling, or by using improved techniques to date museum specimens.

It also means taking into account all the local palaeoenvironmental information that is available to develop a detailed understanding of the palaeoecology of each species and its ecosystem. To reconstruct the diet of an animal, researchers can use stable isotope analyses of tooth enamel. Pollen cores can indicate the local vegetation at the time. The geochemistry of certain formations nearby, such as stalagmites, might give clues about the local climate. Changes in the nature of the sediment laid down in a nearby creek bed, or in the deposition of sand dunes, might hint at local landscape changes. And so on. Broad global palaeo-temperature records are likely to be a crude guide to climatic and environmental changes at local scales.

For each species, investigators should also strive to develop a clearer understanding of the human populations that lived alongside, and the nature of their interactions. This could be obtained by analysing DNA samples extracted from ancient human remains, for instance, or by studying middens, ancient dumps for domestic waste.

For example a study published earlier this year combined new dating approaches with chemical analyses of the bones of the cave bear (Ursus spelaeus), to show that its herbivorous diet had remained unchanged up until its last appearance in Europe, some 23,500 years ago¹⁵. Moreover, cut marks on its bones have revealed that some of these animals were hunted by humans. And researchers have linked the morphology of the extinct eastern African antelope Damaliscus hypsodon to the open, dry grasslands it inhabited, to track the demise of both¹⁶.

Megafaunal fossils can now be dated with much greater efficiency and precision — including those of animals that existed several hundreds of thousands of years ago. This is thanks to various advances, such as combined U-series and electron-spin resonance dating. Other emerging techniques, such as the extraction and analysis of ancient DNA, can shed light on changes to the population size of now-extinct species. Several studies have used such approaches to demonstrate that populations of taxa, from giant Irish elk (Megaloceros giganteus)¹⁷ to the Beringian steppe bison (Bison priscus)¹⁸, plummeted many thousands of years before their ultimate extinction, apparently because of deteriorating local climates and habitat changes.

Some might counter that we’re averse to change and are simply finding another reason to be alarmed about the demise of the field sciences in a digital world¹⁹. But our argument is not with modelling per se. With good data, models could provide crucial insights into large-scale changes and the broad nature of the interactions between humans and other big animals as humans dispersed from Africa. More data, of better quality, can be obtained only through fieldwork and rigorous analysis of fossil materials.

Nature 558, 23-25 (2018)

doi: 10.1038/d41586-018-05330-7