Mylodon darwinii *

During his collecting expeditions in the 1830s in South America, Charles Darwin was fascinated but puzzled by fossil remains of various large, plant eating mammals. These creatures seemed much bigger than any animals currently living in the area. Along with examples of every living animal and bird he could capture, Darwin collected a number of fossils to send back to England, including a large lower jawbone that was eroding out of a costal cliff in Argentina. The anatomist Richard Owen analyzed the jaw and attributed it to a giant ground sloth the size of a hippopotamus, which he dubbed "Mylodon darwinii). ..... we know that it became extinct during the last Ice Age, some 10,000 years ago

Two- and three-toes sloths exist today in South America, but, weighing in at a mere ten or twenty pounds, they are the modest size compared to Mylodon. And unlike Mylodon, both two- and three-toed sloths live in trees. But they seem to have adapted to life in the trees only rather recently in evolutionary terms, since they are rather large for tree-dwelling mammals, not particularly agile aloft, and prefer to descend to the ground for such mundane routines as defecation. A big question was whether the ancestors of the tree sloths had become adapted to the arboreal lifestyle just once, and not particularly gracefully, or whether the two forms of tree sloth were examples of parallel adaptations, whereby ground-dwelling sloths in the past had at least twice independently taken to the trees. If similar adaptations happened independently more than once, -- if history repeated itself, so to speak -- it suggests that there are a limited number of ways in which animals can adapt to an ecological challenge. Each such case of convergence, when two or more unrelated organisms independently evolve similar behaviors or body shapes, is evidence that evolution follows rules -- and is helpful in deducing how these rules work. An example of this was the marsupial wolf that we had studied in Zurich and Berkeley. In the case of tree sloths, just as in the case of the marsupial wolf, we could determine whether convergence has occurred if we could clarify how Darwin's extinct giant ground sloth was related to the two-toed and three-toed tree sloths

I visited the Natural History Museum in London and spent some time there with the amiable curator of Quaternary mammals, Andrew Currant, an expert on mammal paleontology with the build not unlike that of a large Pleistocene mammal. He showed me some of the fossilized bones that Darwin had brought back, and allowed me to cut a small piece from two of the Patagonian Mylodon bones in their collection ........

Once the samples were back in Munich, Matthias Hoss applied his skills to them. As always, I insisted that we first pa in his Mylodon extract and another crude assay to measure how much of that was similar to modern sloth DNA It turned out that about 0.1 percent of the DNA in our best Mylodon bone extract was from the animal itself, the rest having come from other organisms that had lived in the bones after the giant sloth died. This was turned out to be typical of many ancient remains we have since studied.

Focusing on mitochondrial DNA fragments, Matthias managed to use the PCR to reconstruct a stretch of Mylodon mtDNA more than a thousand nucleotide long by amplifying short overlapping pieces. By determining and comparing the same sequences from samples from living sloths, he could show that the giant ground sloth, which stood te feel tall on its hind legs, was more closely related to the present day two-toed tree sloth than to the three toed tree sloth, ...... Pages 63 to 65