One of my favorite evolutionary phenomena to talk (and write) about is convergent evolution. Convergence has occurred whenever phylogenetically distant species, in either or both disparate geographic regions or geological epochs, have evolved very similar phenotypes. This is usually due to similar ecological conditions across the times and places in which the species reside, and in response to similar selective pressures, the different species have in common parallel yet independently evolved adaptations (this concept is illustrated in Figure 1). These similarities are not due to common ancestry and are therefore not homologous. It seems that this would be rare pattern, however the tree of life is full of amazing examples of convergence, and the prevalence of convergence throughout Earth’s history suggests there is at least some degree of determinism in the evolutionary process. I want to write a series of posts which highlight some of my favorite examples of convergent evolution. These posts will try to frame each case in evolutionary, biogeographical, anatomical, and environmental terms.
My first case study in convergent evolution will be the wonderful story of two predatory mammals, which lived at approximately the same time but in separate corners of the globe. One of these is extinct and the other… well it’s complicated. During the Miocene Era which lasted between ~23,000,000 and ~5,200,000 years ago, an environmental change occurred which allowed vast grasslands to open across Eurasia, and with that, the development of a diverse community of crown-toothed herbivorous perissodactyls (“odd-toed” ungulates which include modern-day horses and rhinos) and artiodactyls (“even-toed” ungulates which include modern day cows, sheep and camels) which grazed and browsed across the new prairies. Not surprisingly, there were carnivores which became adapted to chasing down this quick quarry, including canids (the dog family, which includes foxes, jackals and wolves). Most folks are familiar with wolves. Modern wolves (Canis lupus) likely appeared during the early to middle Pleistocene (between ~2,500,000 and 1,000,000 years ago), and are slender, powerful predators adapted to pursuing similarly-sized prey over long distances. They have deep chests and long legs, reminiscent of distance runners, and bone-crunching jaws with pointy canines well-suited for holding onto prey (say, for instance, a fleeing deer), as well as scissor-like carnassial teeth for meat-slicing. No wonder wolves have inhabited a special place in the human psyche – with results that were often bad for wolves. I love wolves, and as a teenager I was mesmerized by reading about their natural history as well as accounts of human-wolf interactions (which often bordered on the fictional), such as “Lobo the King of Currumpaw” by Ernest Thompson Seton in Wild Animals I Have Known (1898) (Figure 2), and especially that of “Thinking Like a Mountain” by Aldo Leopold in A Sand County Almanac (1949), where Leopold recalls the moment he changed from wolf-hater/hunter to one of the pioneers of the environmental and conservation movements. If you ask me, a better appeal for whole-ecosystem conservation has never been written:
We reached the old wolf in time to watch a fierce green fire dying in her eyes. I realized then, and have known ever since, that there was something new to me in those eyes – something known only to her and to the mountain. I was young then, and full of trigger-itch; I thought that because fewer wolves meant more deer, that no wolves would mean hunters’ paradise. But after seeing the green fire die, I sensed that neither the wolf nor the mountain agreed with such a view.
Let’s turn back the clock and take a trip to the the land down under, where after the breakup of the supercontinent Gondwana during the Jurassic Period ~200,000,000 years ago, the resident life forms were stranded at sea on an island landmass known as Australia. Placental mammals had not yet dispersed globally, and the only members of the furred kind in Australia at the time of the continental breakup were the pouched marsupials – and it remained that way until very recently (when humans brought their rats and sheep). During the Miocene, as in Eurasia, grasslands began to open up across Australia, although many regions were still heavily forested. This created many new and unique niches that allowed the resident marsupials to adapt and radiate into a wide variety of forms, such as diprotodonts (which include arboreal possums, familiar and bounding kangaroos, and the extinct large lumbering herbivore Diporotodon), and dasyurids which include the Tasmanian devil and Tasmanian wolf (Figure 3). Wolf? Yes, there is a marsupial wolf. How can that be? The answer is: CONVERGENCE. The scientific name of the Tasmanian wolf is Thylacinus cynocephalus, which means “pouched animal with the head of a dog”. Another more unique and accurate common name is the thylacine.
Thylacines appeared in Australia during the Miocene, and the modern species was present in both Australia and Tasmania by the Pleistocene and into the Holocene, although it was just about absent from mainland Australia by the 1800s and restricted to Tasmania by the 20th century. Just as wolves across Eurasia and North America, thylacines were looked upon as threats to ranchers and were accused of killing chickens and sheep. As a result, they were hunted to extinction and the last one died in a zoo in the Hobart Zoo in Tasmania in 1938. The thylacine was a dog-sized predator, and while its method of prey acquisition is still under debate (whether it was a pursuit predator like wolves or more of an ambush predator like a cougar), together with canids it represents an exceptional case of convergent evolution, which is most evident in a comparison of the cranial anatomy with C. lupus (Figure 4). Both species have an elongated snout with pointed canines and scissor-like molars which are well suited for snapping up, killing and dismembering prey. You’d be hard-pressed to pick one or the other out of a line-up. One of the few cranial traits that can be used as a clue is that the thylacine has more teeth overall, including triangular-shaped molars, thus betraying its marsupial ancestry. But you’d have to be an expert to notice that. While they lived on opposite ends of the world, placental canids and marsupial thylacines developed into similar ecological roles and responded with a convergent evolution toward extremely similar phenotypes.
As mentioned above, while thylacines and wolves evolved into similar forms, they also were met with similar antagonism by humans. Alas, the thylacine died out before the conservation movement took hold. With their much wider distribution across the Northern Hemisphere, placental wolves were able to hold in many areas and were more recently aided by human efforts to bolster their recovery. As Aldo Leopold predicted, the reintroduction of wolves into Yellowstone National Park was a major triumph for conservation, as it returned a keystone species to an ecosystem which was out of equilibrium due to its absence. The return of the thylacine would require either a seance or de-extinctioning through DNA cloning. The probabilities of either of those producing a living thylacine anytime soon are close to zero.