On July 1, 160 years ago (1858) Darwin and Wallace presented their theory on the evolution of species by the action of natural selection at the Linnean Society of London. Since then, many of the animals and plants they discovered and analyzed on their voyages and in their writings have been the object of numerous studies that have clarified their evolutionary significance.
But some animals discovered or analyzed by Darwin and Wallace have refused to reveal their nature, even though they played an important role in the construction of their theory. Among them are two: the Macrauchenia, a strange, extinct giant mammal from South America found by Darwin; and the Babirusa, an unusual pig (endemic to some islands in the Malaysian archipelago) analyzed by Wallace. The Macrauchenia is one of the most “chimerical” animals ever; while the Babirusa is an animal with an extremely strange anatomical feature. The application of a number of genetic and molecular techniques is now at last clarifying their nature.
Darwin’s extinct monsters
In 1834 Darwin discovered the fossilized bones of a strange giant mammal in Argentina and Uruguay during his voyage in the Beagle. This discovery, as well as that of other fossils of large South American mammals such as glyptodonts, Megatheria and mylodons (related to the sloths or armadillos currently alive in these regions), helped him begin to think for the first time of the theory of evolution by natural selection, given that they demonstrated that species can vary over time and even become extinct.
At Darwin’s urging, Richard Owen, the most important zoologist of the time, christened this strange fossil as Macrauchenia, above all because he thought that its long neck and the form of its body meant it was a large Camelid related to the South American llamas.
In fact, Macrauchenia means “long-necked”, with auchenia referring to the genus in which all the South American camelids (llamas, vicuñas, guanacos, alpacas, etc.) were included at the time. But later, since they also had features of other animals, like the long neck of a giraffe, and a skull and strange trunk similar to an elephant, their taxonomical position has been much debated.
Recent molecular studies of these animals have begun to resolve the enigma. In 2015, based on the fossilized bones of these animals that are more than 10,000 years old, an almost complete sequence has been obtained of the protein collagen, the main protein in the skin and bones of mammals (1); and in 2017, the almost complete mitochondrial DNA (2) sequence. Comparing these sequences with those of the rest of the ungulates (the group of mammals with hoofs to which they belong) proves that these mammals have a common ancestry with horses, hippopotami and tapirs, but are not related to camelids, elephants or armadillos and sloths. In addition, the molecular data show that the ancestors of species and groups such as Macrauchenia originated more than 60 million years ago, possibly in the American section of the great land mass of Gondwana, and that later, when the South American continent separated, evolved and proliferated there independently until some 10,000 years ago, when they became extinct – or maybe were made extinct by humans?
In the end, its strange mixture of morphological characteristics could be explained by convergent evolutionary processes, which are parallel with but independent of other mammals such as elephants, giraffes, camelids, etc., with which they share some of their features.
Wallace’s fantastic pig
In his trips around the Malaysian Archipelago, which began in 1854 and lasted 8 years, when he reached the Celebes-Sulawesi Islands (currently part of Indonesia), Wallace mentioned a strange endemic species of pig: the babirusa. According to him, it was a species that evolved from the pigs that arrived from the Asian continent to these islands at a time when the sea level had fallen (due to phenomena such as glaciations, telluric movements, etc.). But when the sea level later rose, the ancestors of the babirusa may have become isolated on the Celebes, thus evolving in a different way from the ancestral species of pigs from which they came.
Since then the babirusa has been an enigma in various ways. It is a strange wild pig for a number of reasons, not only because of its appearance, which is different from other pigs from snout to trotters. In fact, in some cases it has been linked more to the hippopotamus than to pigs.
In particular, what is characteristic and practically unique in this animal is that instead of growing downward as is normal, its upper canines grow upward and backward, and even touch its snout and sometimes perforate it. Given that there are deer on the Celebes islands that have developed tusks outside the snout (but never backward and never to such an extent), the natives gave them the name babirusa, or “pig-deer”.
Little progress has been made on the origin and evolutionary meaning of these strange tusks since Wallace’s time, although some folk explanations have survived: one native legend has it that the animals used them to hang from the trees while they slept and thus escape their predators. At any rate, if they do serve any purpose, it is perhaps as protection for the snout when they fight, or even as an indicator of sexual prowess and the health status of the males.
In contrast, numerous genetic and molecular studies are now examining Wallace’s hypothesis on the origin of this species in depth. The sequencing of various genes and nuclear and mitochondrial proteins and even the complete genomes of the babirusa and other Suidae (a group that also includes a number of Asian/European species of pigs, wild boars, and other African species such as warthogs, giant forest hogs, red river hogs, etc.) have demonstrated that this is the most differentiated of the Suidae: its ancestors separated earliest from the rest of its group in evolutionary terms – in fact, even before the divergence between the Eurasian and African species (3).
At the same time, these studies provide us with data that explain other biological characteristics that differentiate the babirusa from other pigs, such as its low reproductive rate or the different way its digestive, reproductive or cardiac systems are structured and operate. But that’s another story, in which it’s also worth including two other points: that the babirusas are depicted in cave paintings on the Celebes Islands dating back to nearly 40,000 years ago (4); and that thanks to modern techniques of reproduction and animal care it has been possible recently to reproduce a babirusa in a zoo (San Diego, USA), which opens up interesting possibilities in the fight to prevent its extinction.
Manuel Ruíz Rejón
- Welker, F. 2015. Ancient proteins resolve the evolutionary history of Darwin’s South American ungulates. Nature .Jun 4, 522 (7554):81-84.
- Westburg M. et al. 2017. A mitogenomic time tree for Darwin enigmatic South American mammal Macrauchenia patachonica. Nature Commun. Jun 27, 8: 15951.
- Frantz, L. et al. 2016, The evolution of Suidae. Rev. Anim. Biosci. 4: 61-85.
- Aubert,M. et al. 2014. Pleistocene cave art from Sulawesi, Indonesia. Nature, Oct. 9, 514 (7521): 223-227.