Genetics of South American rodents points to evolution

Rodents are an extremely successful group of mammals. There are more species of rodents than any other type of mammal, and they inhabit nearly every stretch of land on earth.

Some rodents are geographically restricted, however, with a number of groups being located entirely in the Americas. Chinchillas and viscachas (Chinchillidae) are exclusively found in South America, as is the pacarana (Dinomyidae), chinchilla rats (Abrocomidae), degus and kin (Octodontidae), tuco-tucos (Ctenomyidae), the coypu (Myocastoridae), and cavies (Caviidae). Spiny rats (Echimyidae), agoutis (Dasyproctidae), capybaras (Hydrochoeriidae) and the paca (Cuniculidae) live in South America but have also ventured into Central America. American porcupines (Erethizontidae) occupy both North and South America, and the hutias (Capromyidae) only live on Caribbean islands.

According to genetics, however, these rodents aren’t simply friendly neighbors: they’re relatives! Molecular phylogenetics has routinely shown that these rodents are all more genetically similar to each other than they are to other rodents, implying that they descended from a common ancestor. This is despite these rodents encompassing species with very different adaptations, including runners (maras, agoutis), tree-dwellers (porcupines), burrowers (coruro, tuco-tucos), swimmers (coypu, capybaras), and some species have particularly spiny hairs (spiny rats, porcupines).

Below is a phylogeny [1] that compared 35,603 letters of DNA between 164 different species of mammals, showing this very result. Click and zoom in on the figure and you’ll see the rodents among the blue branches, more than half way down. Use the capybara painting to help orient yourself, and refer to the family names I introduced above (e.g., Chinchillidae, Caviidae) to help you find all of these rodents in this genetic family tree.

Mammal phylogeny
Mammal molecular phylogeny

So given that these rodents are more genetically similar to each other than to other rodents, and they predominantly live in South America, it suggests that an ancestral species came to South America and diversified into a wide array of forms.

But is it that simple? What does the fossil record say? As you can see below [2], with the exception of a few dubious 56–33.9 million year old spiny rat fossils in East Asia, each of these families of rodents is completely restricted to the Americas. The oldest fossils are found in South America (33.9–28.1 million years ago), before ultimately reaching the Caribbean (20.44–15.97 million years ago) and North America (1.8–0.78 million years ago). Click on the image below to get a closer look.

south american rodent fossil distribution
South American rodent fossil distribution

How those first rodents got to South America is another question, but perhaps they would be proud to know that their great-great-great-etc. grandchildren successfully conquered a continent in an array of different forms.

Questions for Creationists

Why are all of these American rodents more genetically similar to each other than to other rodents? Are they all part of the same ‘kind’ and just recently evolved into these various forms? If so, is this degree of evolution, leading to forms adapted to running, burrowing, having spiny quills, etc. consistent with Creationism? If they are different kinds, why did they all go to the Americas together? And if they did, is it just a coincidence that they are genetically similar to one another? If these ‘kinds’ were all on Noah’s ark, shouldn’t we find fossils of these animals elsewhere in the world? How did they cross the Atlantic Ocean? If humans brought them, why are they all genetically similar?

References

1. Meredith, R. W., Janečka, J. E., Gatesy, J., Ryder, O. A., Fisher, C. A., Teeling, E. C., … & Murphy, W. J. (2011). Impacts of the Cretaceous Terrestrial Revolution and KPg extinction on mammal diversification. Science, 334(6055), 521-524.

2. Paleobiology database

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