Evolution occurs when an environmental change causes an organism to alter the way they live and evolve into a more well adapted version of their predecessor. But environmental changes, behavioral alterations, and anatomical modifications are far from synchronous in a species’ evolution. A new study in Nature journal uses elephants and their ancestors as evidence that evolution isn’t a predictable, step-by-step process.
Adrian M. Lister from the Natural History Museum in London looked at fossils of proboscideans—the order including trunked mammals, of which elephants are the only extant family—from a period of time spanning 20 million years, hoping to demystify the timing of evolution in these creatures.
Lister examined the tooth enamel from a number of proboscideans to determine what different species from various time periods were feeding on. Over time, what an animal eats changes the chemical composition of their tooth enamel. When teeth fossilize, they retain certain chemical remnants that tell scientists what it is they were feeding on—lending credence to the well-known adage: you ARE what you eat.
10 million years ago, the climate of East Africa became drier and cooler, and the woodlands began transforming into large spans of grass, resulting in widespread prairie land such as you might find there today. 2 million years later, elephant predecessors were beginning to turn their trunked heads downward—from eating leaves off trees and shrubs, to feeding on the long grasses of the Africa prairielands. But they weren’t eating effectively—they didn’t have teeth that were well adapted to grazing—at least not yet.
5 million years ago, elephantid tooth enamel started looking different: the crown height of their teeth increased by threefold, and the number of enamel lamellae—small cracks and indents in the tooth—increased. Both of these changes are considered to be adaptations to a grass-dominated diet. Elephants were finally catching up with their new environment.
A fossil tooth belonging to Elephas antiquus, an elephantid species that lived between 780 and 50 million years ago.
It took a good 2 million years for elephantid lineages to register the environmental change and subsequently alter their behavior—and another 3 for the selective pressure for those with grazing ability to change their morphology accordingly. Elephants were grazing before they had the tools to graze.
Anatomical changes lagging behind behavioral ones is a phenomena scientists hadn’t necessarily considered or previously thought to be true.“Such offsets between behavior and morphology demand that we think beyond the traditional paleontological assumption of a lock-step between the two.” Lister explains in the study.
The study is giving scientists a deeper understanding of the selective forces responsible for the process by which a species evolves over time, and how behavior dictates that process. When the environment changes, an organism doesn’t automatically alter its behaviors, and in turn undergo morphological transformations. The process of evolution takes a long time and the order in which changes occur isn’t necessarily as straightforward as we once thought. This finding will help scientists better distinguish different selective forces and how they affect evolution.
Lister’s elephantid research was reported June 26, 2013 in the journal Nature.
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