Fallback foods are the foods that an organism eats when it can’t find the good stuff. It has been suggested that adaptive changes in fallback food strategies can leave a more distinct mark on the morphology of an organism, including in the fossil record, than changes in preferred food strategies. This assertion is based on work done by the Grants and others with Galapagos Island finches, by Richard Wrangham and me with hominids, and by Betsy Burr and me with rodents.
The reason for this is simple. There is a rough correspondence between how much energy one can obtain from a food type and whether or not it is a preferred vs. fallback food. Fruit contains lots of energy available to, say, a mammal, while bark contains less. There is also a rough correspondence between how much work one has to do in terms of mastication (chewing) and digestion (like, fermentation for bark and leaves as opposed to relatively low-cost absorption for sugars) to get the energy that is there.
It is therefore likely that one will see strong selection for changes in anatomy of chewing and digestion if a population experiences an increased reliance on the harder to get energy from fallback foods.
In addition, it is more likely that a shift between two fundamentally different kinds of fallback foods will cause an obvious change in dietary adaptations, while a shift in primary foods may involve a less obvious change. Richard Wrangham and I think that the change from a presumably chimpanzee-like ancestor of humans and chimps to the australopithecines (early hominids) is exactly such a change.
Apes normally eat fruit as a primary food, and leaves as a fallback food. This is known from behavioral observations in the field, and is in accord with dietary anatomy (apes are “built” to seek, find, masticate, and digest fruit, but also, leaves). Austrlalopithecines, however, have very different teeth and associated masticatory apparatus (the musles and bones that make the teeth work). The chewing system of these early hominids looks nothing like any known system for earting leaves, but it does look a lot like a known system for eating something else that counts as a fallback food … roots.
Wrangam and I have shown that roots are commonly eaten by human foraging populations, that roots are more abundant in the kinds of habitat we believe early Australopithecines lived in, that roots became a more abundant food type in tropical Africa at about the same time that Australopithecines diversified, and that root eating rodents also spread and diversified at the same time. Subsequently, Burr and I (mainly Burr … she did all the hard work) have shown that there is a specific suite of root-related morphological adaptations found in root eating rodents, to different degrees and in slightly different ways, in several different rodent groups. Most interestingly, this suite of adaptations is essentially the same as what we see in the australopithecines.
A forest ape (a chimpanzee-like common ancestor of Australopithecus and Homo on one hand, and living chimps on the other) would have eaten leaves as its main fallback food. If dry spells reduced the availability of fruit, these animals would switch to leaves, but this would require not only staying in forest habitats, but also retreating from relatively dry forest margin, as the leaves found in these habitats are less edible. But if roots were part of the fallback diet for some of these groups, dry conditions that would reduce fruit would not force them to retreat from forest margins. Rather, those groups living near forest margins would benefit from heading periodically out of the forest into adjoining savannas, to obtain roots. This is because roots are rare in forests, more common along forest margins, and even more common out in the savanna. Generally speaking, the dryer the environment (in the African tropics) the more roots one can find.
In a sense, roots as fallback foods act as a moving walkway that switches on now and then and moves forest apes into savanna habitats. We feel that this was the key (although certainly not only) evolutionary event leading to the chimp-human split.
LADEN, G., WRANGHAM, R. (2005). The rise of the hominids as an adaptive shift in fallback foods: Plant underground storage organs (USOs) and australopith origins. Journal of Human Evolution, 49(4), 482-498. DOI: 10.1016/j.jhevol.2005.05.007