How plants helped us evolve

style="float: right; margin-bottom: 10px; font-weight: 600;"Sun 23rd Dec, 2012

A lesser known hypothesis suggesting that our relationship with fruit may have helped us evolve into humans, gains new support. According to the theory, known as the angiosperm coevolution hypothesis, long ago, our primate ancestors evolved key adaptations like forward-facing eyes, excellent colour vision, rounded, blunt teeth and fingers without claws, all for the purpose of eating and living from fruits.

The most widely accepted hypothesis proposed to explain how primates evolved from other mammals is the so-called "visual predation" hypothesis, originally proposed by Dr. Matt Cartmill in 1974. The hypothesis suggests that large, forward-facing eyes and grasping hands evolved in primates to allow them to eat insects in trees. Their large eyes would have helped them spot insects in the dark, and their grasping hands would have allowed them to climb trees and catch insects with skill.

Now, a new report by Drs. Robert Sussman and Tab Rasmussen of the University of Washington (St. Louis, Missouri, USA) and Dr. Peter Raven of Missouri Botanical Garden published in the American Journal of Primatology on November 26, brings together current evidence supporting the angiosperm hypothesis, makes a compelling case for its consideration.

After spending endless hours watching lemurs - a primitive group of primates - eating fruit in Madagascar, Drs. Sussman and Raven came up with the angiosperm co-evolution hypothesis. Fruit itself only really developed around the start of the rise of mammals in the Eocene, after the dinosaurs had died out. Drs. Sussman and Raven conjectured that: "As angiosperms...began to evolve into tropical forests, they became bigger and bigger and so did their seeds. They needed large animals to spread these seeds...edible fruits developed as a means of attracting large animals for this dispersal... three types of animals co-evolved with these large tropical angiosperms...: frugivorous bats, frugivorous birds, and primates.".

This new report draws together several lines of evidence supporting the angiosperm co-evolution hypothesis (and going against the visual predation hypothesis). The main evidence is that the "dental structure and the visual system of primates both seem to be best understood as adaptations for eating fruits and for seeing through a cluttered, leafy environment, rather than for hunting and eating insects... [and a recently found early primate fossil], ... Carpolestes simpsoni, that has dentition for eating fruits and that has primate-like grasping extremities, but not forward facing eyes, seems to indicate that these early...primates were attempting to exploit fruit ...[at the ends of tree] branches initially, and later developed better and better ability to see through leafy environments. Finally, this [hypothesis]... better fits the chronology of primate evolution" as the diversification of primates into lots of species corresponds in time to the diversification of angiosperms.

But is this evidence enough? After all, the most recent piece of evidence comes from a single fossil, Carpolestes simpsoni. Dr. Sussman believes that it is: "although there is only one particular fossil that fits the theory specifically, there is a great deal of other evidence and context that supports the hypothesis. In fact, no other hypothesis so far can fit both the fossil evidence, the coincidence of the evolutionary timing, the fact that primates were evolving at the same time as other angiosperm-feeding taxa of animals, and the new data on X-Ray vision among primates and its function.". "if forward-facing eyes evolved at the same time as insect-feeding adaptations of the teeth, and grasping extremities... this would give pretty substantial support for the visual ...predation hypothesis. However, this is why C. simpsoni is so important. Also, primate feeding adaptations, in general, are essentially plant-eating adaptations, both in the dentition and in the gut tract, and not adaptations for eating animals protein.".

So, it seems primates evolved a fruit eating habit, and Dr. Sussman believes that "as we learn more about our ancestors, we begin to understand the building blocks of what made us what we are. ...the earliest human ancestors were likely animals that were hunted rather than ones that were hunters. This a very different picture of our evolutionary adaptations and our basic nature. For example, ...our social structure and behaviour [probably] developed protect one another from predators rather than to be successful in being [predators]". We are then, it seems, originally gentle fruit eaters, protecting each other from harm, rather than coordinated pack-hunters. Unfortunately or fortunately, we also became rather good at hunting.

SUSSMAN, R. W., TAB RASMUSSEN, D. and RAVEN, P. H. (2012), Rethinking Primate Origins Again. Am. J. Primatol.. doi: 10.1002/ajp.22096

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