A brief history of Proboscideans

Lyuba, the best preserved mammoth mummy in the world, at the Field Museum of Natural History (From Wikimedia Commons).

In 1811, German zoologist Johann Karl Wilhelm Illiger introduced the taxonomic order Proboscidea for elephants, the American mastodon and the woolly mammoth. The proboscis, an elongated appendage from the head of an animal, is the most distinguishing feature of these mammals. They also have a highly specialized dentition, and tusks that formed from elongated upper incisors. The lineage arose in the Late Paleocene in Africa and spread across Eurasia and the Americas. Over their 60 million years of evolutionary history, proboscideans went from a few kilograms in the earliest representatives, to forms weighed up to 6-8 metric tons. Phosphatherium escuilliei, one of the earliest recognized proboscidean, stood about 30 centimetres with a body mass of 17 kilograms while Palaeoloxodon recki stood 4.27 metres tall and weighed 12.3 tonnes. Today, the clade is represented by only 3 species: the African forest elephant, Loxodonta cyclotis, the African bush elephant, Loxodonta africana and the Asian elephant, Elephas maximus.

Skull and upper dentition of Eritherium azzouzorum, the oldest and most primitive elephant relative. From Gheerbrant 2009

Traditionally, three major radiations have been recognized. The first radiation occurred between Paleocene-Oligocene and was restricted to Afro-Arabia. The second radiation involved the expansion of taxa that emerged between the Late Oligocene and the early Miocene outside Africa. The third radiation emerged at the end of the Miocene and extended through the Holocene epoch. The most primitive and smallest known proboscidean belong to the family Plesielephantiformes. The most diverse family was the Gomphotheriidae, which lived on all continents except Antarctica and Australia. The earliest Elephantiformes were similar in appearance to the first gomphotheres. The iconic mammoths were widespread in the northern hemisphere during the Last Ice Age and their remains inspired all types of legends.

Gomphotherium angustidens at Senckenberg Museum of Frankfurt. From Wikipedia Commons

The onset of C4 grass-dominated habitats around 8 Ma brought dramatic changes to the evolutionary context of megafauna communities. The adaptation to particular feeding habits is manifested in changes to the upper and lower incisors of proboscideans. Proboscideans in the first radiation were mostly browsers, whereas those in the second and third radiations were mostly grazers. Miocene forms, such as Gomphotherium angustidens and Rhynchotherium tlascalae are known to have slightly more hypsodont molars than Paleocene–Oligocene proboscideans.

A mammoth tooth on the riverbank on Wrangel Island. Image credit; Juha Karhu/University of Helsinki

In Europe, during the Pliocene, took place the extinction of the Deinotheriidae (they survived in Africa until the early Pleistocene), Mammutidae, and Gomphotheriidae. The Pliocene also witnessed the rise of stegodonts (Stegodontidae) and modern elephants (Elephantinae). During the Pleistocene, continental glaciars expanded and contracted over most of northern hemisphere causing dramatic ecological shifts. Most of the terrestrial megafauna became extinct. The extinction was notably more selective for large-bodied animals than any other extinction interval in the last 65 million years (dwarfed mammoths survived until 4000 years ago on Wrangell Island). Today, the greatest threat to elephants is loss of habitat and poaching for the illegal ivory trade.

 

References:

Cantalapiedra, J.L., Sanisidro, Ó., Zhang, H. et al. The rise and fall of proboscidean ecological diversity. Nat Ecol Evol (2021). https://doi.org/10.1038/s41559-021-01498-w

Gheerbrant, E (2009). “Paleocene emergence of elephant relatives and the rapid radiation of African ungulates”. Proceedings of the National Academy of Sciences of the United States of America. 106 (26): 10717–10721. doi:10.1073/pnas.0900251106.

Shoshani, J. (1998). Understanding proboscidean evolution: a formidable task. Trends in Ecology & Evolution, 13(12), 480–487. doi:10.1016/s0169-5347(98)01491-8 

Body and brain size evolution in genus Homo

 

Neanderthal skull (Image credit: Halamka/Getty Images)

Almost 2 million years ago in East Africa, hominin diversity reached its highest level with the appearance of the robust Paranthropus species, as well as the first specimens attributed to the genus Homo. This period is also marked by a dramatic increases in hominin body and brain size. Several theories have been developed to explain the interaction between African paleoclimate and early hominid evolution. The savannah hypothesis suggested that hominins were forced to descend from the trees and adapted to life on the savannah facilitated by walking erect on two feet. This idea was already outlined by Lamarck in his Philosophie zoologique (1809], where he describes in details how an early ancestor of primeval human abandons an arboreal life to adapt itself to open plains. More recently, the pulsed climate variability hypothesis highlights the role of short periods of extreme climate variability specific to East Africa in driving hominin evolution and subsequent dispersal events.  Now, a new study conducted by an interdisciplinary research team from Cambridge University and Tübingen University tested the influence of environmental factors on the evolution of body and brain size in the genus Homo over the last one million years.

Location and sample size (n) of body (squares) and brain size (triangles) estimates for individual Homo fossils used in the study by Will, M., Krapp, M., Stock, J.T. et al. 2021.

In the study, the team combines data from more than 300 fossils of the genus Homo divided into three taxonomic units: Mid-Pleistocene Homo, Homo neanderthalensis, and Pleistocene Homo sapiens distributed over the Old World. The environmental information for each fossil comes from a climate emulator (GCMET) that takes into account long-term, glacial-interglacial climate variation, caused by changes in the Earth’s orbit around the sun and in greenhouse gases.

The team found that temperature is a major predictor of body size variation, with larger-bodied individuals consistently occurring in colder climates. This increase in body size with decreasing environmental temperature is consistent with the Bergmann’s rule and could be explained because heat is dissipated more slowly in larger animals as the surface-area to volume ratio diminishes, so it would be a thermal advantage in colder habitats. They also found that brain size within Homo is less influenced by temperature suggesting that body and brain size are under different selective pressures.

 

References:

Will, M., Krapp, M., Stock, J.T. et al. Different environmental variables predict body and brain size evolution in Homo. Nat Commun 12, 4116 (2021). https://doi.org/10.1038/s41467-021-24290-7

Maslin M.A., C. Brierley, A. Milner, S. Shultz, M. Trauth, K. Wilson “East African climate pulses and early human evolution” Quaternary Science Reviews (2014). DOI:10.1016/j.quascirev.2014.06.012

Shultz S, Maslin M (2013) Early Human Speciation, Brain Expansion and Dispersal Influenced by African Climate Pulses. PLoS ONE 8(10): e76750. DOI: 10.1371/journal.pone.0076750