Global Climate Dynamics Drove the Decline of Mastodonts and Elephants, Not Overhunting by Early Humans

Dusk Falls on East Africa’s Turkana Basin

Dusk falls on East Africa’s Turkana Basin 4 million years ago, where our early upright-walking ape ancestors, Australopithecus anamensis (foreground), shared their habitat with several coexisting proboscidean species, as part of a spectacular herbivore community containing some progenitors of today’s charismatic East African animals. Background (left to right): Anancus ultimus, last of the African mastodonts; Deinotherium bozasi, colossal herbivore as tall as a giraffe; Loxodonta adaurora, gigantic extinct cousin of modern African elephants, alongside the closely-related, smaller L. exoptata. Middle ground (left to right): Eurygnathohippus turkanense, zebra-sized three-hoofed horse; Tragelaphus kyaloae, a forerunner of the nyala and kudu antelopes; Diceros praecox – ancestor of the modern black rhino. Credit: Julius Csotonyi

Elephants and their forebears were pushed into wipeout by waves of extreme global environmental change, rather than overhunting by early humans, according to new research.

The study, published today (July 1, 2021) in Nature Ecology & Evolution, challenges claims that early human hunters slaughtered prehistoric elephants, mammoths, and mastodonts to extinction over millennia. Instead, its findings indicate the extinction of the last mammoths and mastodonts at the end of the last Ice Age marked the end of progressive climate-driven global decline among elephants over millions of years.

Fossil Skull of Typical Mid Miocene Shovel Tusker

Highly complete fossil skull of a typical mid Miocene ‘shovel-tusker’, Platybelodon grangeri, roamed in large herds across Central Asia 13 million years ago. The specimen is display mounted at the Hezheng Paleozoological Museum, Gansu Province, China. Credit: Zhang Hanwen

Although elephants today are restricted to just three endangered species in the African and Asian tropics, these are survivors of a once far more diverse and widespread group of giant herbivores, known as the proboscideans, which also include the now completely extinct mastodonts, stegodonts, and deinotheres. Only 700,000 years ago, England was home to three types of elephants: two giant species of mammoths and the equally prodigious straight-tusked elephant.

An international group of paleontologists from the universities of Alcalá, Bristol, and Helsinki, piloted the most detailed analysis to date on the rise and fall of elephants and their predecessors, which examined how 185 different species adapted, spanning 60 million years of evolution that began in North Africa. To probe into this rich evolutionary history, the team surveyed museum fossil collections across the globe, from London’s Natural History Museum to Moscow’s Paleontological Institute. By investigating traits such as body size, skull shape, and the chewing surface of their teeth, the team discovered that all proboscideans fell within one of eight sets of adaptive strategies.

“Remarkably for 30 million years, the entire first half of proboscidean evolution, only two of the eight groups evolved,” said Dr. Zhang Hanwen, study coauthor and Honorary Research Associate at the University of Bristol’s School of Earth Sciences.

“Most proboscideans over this time were nondescript herbivores ranging from the size of a pug to that of a boar. A few species got as big as a hippo, yet these lineages were evolutionary dead-ends. They all bore little resemblance to elephants.”

Global Climate Dynamics Drove the Decline of Mastodonts and Elephants

A scene from northern Italy 2 million years ago – the primitive southern mammoths Mammuthus meridionalis (right-hand side) sharing their watering hole with the mastodont-grade Anancus arvernensis (left-hand side), the last of its kind. Other animals that brought an ‘East African air’ to Tuscany included rhinos, hippos and zebra-like wild horses. Credit: Tamura Shuhei

The course of proboscidean evolution changed dramatically some 20 million years ago, as the Afro-Arabian plate collided into the Eurasian continent. Arabia provided crucial migration corridor for the diversifying mastodont-grade species to explore new habitats in Eurasia, and then into North America via the Bering Land Bridge.

“The immediate impact of proboscidean dispersals beyond Africa was quantified for the very first time in our study,” said lead author Dr. Juan Cantalapiedra, Senior Research Fellow at the University of Alcalá in Spain.

“Those archaic North African species were slow-evolving with little diversification, yet we calculated that once out of Africa proboscideans evolved 25 times faster, giving rise to a myriad of disparate forms, whose specializations permitted niche partition between several proboscidean species in the same habitats. One case in point being the massive, flattened lower tusks of the ‘shovel-tuskers’. Such coexistence of giant herbivores was unlike anything in today’s ecosystems.”

Extinct Proboscideans National Museum of Natural History Paris

The gallery of extinct proboscideans in the Muséum national d’Histoire naturelle, Paris, echoing their bygone golden age. Credit: Zhang Hanwen

Dr. Zhang added: “The aim of the game in this boom period of proboscidean evolution was ‘adapt or die’. Habitat perturbations were relentless, pertained to the ever-changing global climate, continuously promoting new adaptive solutions while proboscideans that didn’t keep up were literally, left for dead. The once greatly diverse and widespread mastodonts were eventually reduced to less than a handful of species in the Americas, including the familiar Ice Age American mastodon.”

By 3 million years ago the elephants and stegodonts of Africa and eastern Asia seemingly emerged victorious in this unremitting evolutionary ratchet. However, environmental disruption connected to the coming Ice Ages hit them hard, with surviving species forced to adapt to the new, more austere habitats. The most extreme example was the woolly mammoth, with thick, shaggy hair and big tusks for retrieving vegetation covered under thick snow.

The team’s analyses identified final proboscidean extinction peaks starting at around 2.4 million years ago, 160,000 and 75,000 years ago for Africa, Eurasia, and the Americas, respectively.

Disparity of Proboscidean Forms

Disparity of proboscidean forms through 60 million years of evolution. Early proboscideans like Moeritherium (far left) were nondescript herbivores typically the size of a pig. But subsequent evolution of this lineage was almost consistently dominated by gigantic species, many considerably larger than today’s elephants (e.g. Deinotherium 2nd left; Palaeoloxodon furthest right). A key factor of proboscidean evolutionary innovation lies with disparities in tooth morphology. Credit: Óscar Sanisidro

“It is important to note that these ages do not demarcate the precise timing of extinctions, but rather indicate the points in time at which proboscideans on the respective continents became subject to higher extinction risk,” said Dr. Cantalapiedra.

Unexpectedly, the results do not correlate with the expansion of early humans and their enhanced capabilities to hunt down megaherbivores.

“We didn’t foresee this result. It appears as if the broad global pattern of proboscidean extinctions in recent geological history could be reproduced without accounting for impacts of early human diasporas. Conservatively, our data refutes some recent claims regarding the role of archaic humans in wiping out prehistoric elephants, ever since big game hunting became a crucial part of our ancestors’ subsistence strategy around 1.5 million years ago,” said Dr. Zhang.

“Although this isn’t to say we conclusively disproved any human involvement. In our scenario, modern humans settled on each landmass after proboscidean extinction risk had already escalated. An ingenious, highly adaptable social predator like our species could be the perfect black swan occurrence to deliver the coup de grâce.”

Reference: “The rise and fall of proboscidean ecological diversity” by Juan L. Cantalapiedra, Óscar Sanisidro, Hanwen Zhang, María T. Alberdi, José L. Prado, Fernando Blanco and Juha Saarinen, 1 July 2021, Nature Ecology & Evolution.
DOI: 10.1038/s41559-021-01498-w

4 Comments on "Global Climate Dynamics Drove the Decline of Mastodonts and Elephants, Not Overhunting by Early Humans"

  1. Clyde Spencer | July 1, 2021 at 9:31 pm | Reply

    It can be said safely that none of the numerous mass extinctions, and none of even the many individual lineages that were dead ends, could possibly have been the result of humans if humans had not yet evolved. Broadly speaking, it may have been climate changes that were responsible. However, some life forms survived by evolving into other species, such as polar bears have done. There are many things we don’t understand about the extinction process. There is compelling evidence from several lines that dinosaurs were in decline 10 million years before the Great Asteroid hit. Yet, small dinosaurs, that were in direct competition with small mammals, became extinct as well!

    Probably an important factor is the rate of reproduction. Very large animals, and apex predators, tend to have lower reproduction rates than very small prey animals. Therefore, during rapid changes, such as occurred during periods of orogeny, or mountain building, large animals probably have difficulty evolving rapidly enough to keep their lineage alive. But there are still some key pieces missing in the extinction puzzle!

    • Torbjörn Larsson | July 3, 2021 at 9:29 am | Reply

      That is arguably not correct at all!?

      First, we live in what scientists think is the “greath 6th mass extinction”. “The Holocene extinction, otherwise referred to as the sixth mass extinction or Anthropocene extinction, is an ongoing extinction event of species during the present Holocene epoch (with the more recent time sometimes called Anthropocene) as a result of human activity. … The current rate of extinction of species is estimated at 100 to 1,000 times higher than natural background extinction rates.” [ ].

      Second, the scientists tehmselves attribute the final extinctions, which may or may not be included in your definition of mass extinction, to humans (Homo) as we evolved over 3 million years ago, long before anatomically modern humans (evolved over 300,000 years ago).

      Thrd, by causality mass extinctions causes that predates humans are not relevant for whether or not humans cause or contribute. (It is relevant for whether humans are necessary – evidently not.)

      As a nitpick on “small dinosaurs”, small avian dinosaurs survived and diversified to twice the mammalian diversity.

      If you are interested in extinctions and radiations in general, I recommend the recent largest data dive and model of it. I quote the abstract in full, since it is hard to represent the massive data paper well. But I recommend their figure 1 (paper not paywalled) for a pity model. In short they find that (mass) extinction and (mass) radiations are decoupled, but the whole process can be described by a simple decay clock (whatever its cause).

      “The hypothesis that destructive mass extinctions enable creative evolutionary radiations (creative destruction) is central to classic concepts of macroevolution1,2. However, the relative impacts of extinction and radiation on the co-occurrence of species have not been directly quantitatively compared across the Phanerozoic eon. Here we apply machine learning to generate a spatial embedding (multidimensional ordination) of the temporal co-occurrence structure of the Phanerozoic fossil record, covering 1,273,254 occurrences in the Paleobiology Database for 171,231 embedded species. This facilitates the simultaneous comparison of macroevolutionary disruptions, using measures independent of secular diversity trends. Among the 5% most significant periods of disruption, we identify the ‘big five’ mass extinction events2, seven additional mass extinctions, two combined mass extinction–radiation events and 15 mass radiations. In contrast to narratives that emphasize post-extinction radiations1,3, we find that the proportionally most comparable mass radiations and extinctions (such as the Cambrian explosion and the end-Permian mass extinction) are typically decoupled in time, refuting any direct causal relationship between them. Moreover, in addition to extinctions4, evolutionary radiations themselves cause evolutionary decay (modelled co-occurrence probability and shared fraction of species between times approaching zero), a concept that we describe as destructive creation. A direct test of the time to over-threshold macroevolutionary decay4 (shared fraction of species between two times ≤ 0.1), counted by the decay clock, reveals saw-toothed fluctuations around a Phanerozoic mean of 18.6 million years. As the Quaternary period began at a below-average decay-clock time of 11 million years, modern extinctions further increase life’s decay-clock debt.”
      [“Impacts of speciation and extinction measured by an evolutionary decay clock”, Nature 2021.]

  2. John Donohue | July 4, 2021 at 11:23 pm | Reply

    Request for information:

    North America only.
    12,000-18,000 year ago, humans begin arriving in small batches. Largely, they kept going south, where it was warm.
    Question: What were the counts of various megafauna as the ice melted and humans arrived?

    Unless the animals had already been reduced by the Wisconsin or an asteroid collision, it does not pass the smell test to blame the extinction on human predation over the vast Northern Hemisphere. Simply not enough Homo compared to millions of megafauna and the stupendous territory involved.

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