Sink your teeth into dietary evolution

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Mar. 4, 2005. 07:08 AM

Sink your teeth into dietary evolution

Our ancestors ate as chimps do now

Cooking, cutting changed dentition

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PETER CALAMAI

SCIENCE WRITER

 

WASHINGTON—Low-carb proponents say their diet is healthier because it's

close to what our early ancestors ate. Vegetarians make similar claims.

Neither is entirely right, according to current science.

 

" The common ancestor of all hominids was probably eating the same as

modern chimps, " says renowned paleoanthropologist Bernard Wood.

 

And that means lots of fruit year-round (heavy on the wild figs) and

averaging about 65 grams of meat a day, mostly from pigs and monkeys.

 

Wood's verdict came at the end of a symposium on the origin and evolution

of the modern human diet here at the recent annual meeting of the American

Association for the Advancement of Science.

 

It's a tribute to the ingenuity and persistence that researchers like Wood

can come to any conclusions about the diet of our earliest ancestors.

After all, nobody was keeping records of daily caloric intake two to three

million years ago.

 

The anthropologists in the session used these techniques to reconstruct

the ancient dietary picture:

 

Peter Lucas from George Washington University has devoted most of his

career to uncovering why the arrangement of teeth in the human jaw is such

a mess. The eating habits of our earliest ancestors played a big role.

 

Craig Stanford from the University of Southern California has spent eight

years studying the daily lives of chimpanzees and gorillas in a small

patch of Uganda tropical forest. Their interaction provides a window into

what could have taken place when two different species of early humans

shared the Rift Valley, including how their diets evolved differently.

 

Peter Ungar from the University of Arkansas has enlisted leading-edge

technologies to look at fossil teeth from early hominids and determine

what the teeth were intended to eat and what they actually did eat.

 

Teeth are crucial in reconstructing the diet of hominids and other early

primates because they're the only part of the digestive system that

survives for study.

 

By looking at the teeth of living primates, researchers have found that

those whose diet consists of leaves and insects have long crests on their

molars to shear and slice those tough foods. The apes and monkeys who eat

mostly nuts and seeds, by contrast, have blunter molars with shorter

crests so they can crush and grind such hard, brittle foods.

 

Yet the surviving fossil teeth from many distant primates, including

several likely human ancestors, are often so worn that standard methods

for measuring and comparing the shape of the crests don't work. Enter

topographic analysis, a new technique that produces a 3-D " map " by

combining high-resolution laser scanning with software from Global

Information Systems (GIS).

 

Ungar applied this powerful new tool to worn molars from two types of

hominids that lived at different times in the Sterkfontein valley of South

Africa, near what is now Johannesburg. The hominid closer to the human

ancestral line, called Australopithecus africanus, had noticeably steeper

molar slopes than a near-cousin Paranthropus robustus, whose heavy

crushing molars were attuned to a diet rich in roots and seeds, the foods

of the African savannah.

 

When wide climate swings triggered major shifts in the African

environment, Paranthropus was too specialized to adjust. Instead the

evolutionary nod went to the Australopithecus line, whose adaptability

included teeth capable of handling a greater variety of foods.

 

" Our own ancestors broadened their subsistence base rather than focusing

narrowly, as did their Paranthropus cousins. We had the ability to exist

in a broad range of environments, allowing us find something to eat just

about anywhere we went, " Ungar said in a follow-up email this week.

 

These two kinds of hominids occupied the same region, but at times a

half-million years apart. Craig Stanford has been investigating how diet

may have evolved when two hominids lived at the same time within spitting

distance of one another, as the fossil evidence suggests happened 2 1/2

million years ago in the Rift Valley.

 

Stanford's laboratory is 25 square kilometres of tropical forest in

southwestern Uganda, in the evocatively named Bwindi Impenetrable National

Park. The area is within sight of the region in neighbouring Rwanda where

the late Dian Fossey worked for two decades (Gorillas in the Mist) but a

thousand metres lower in elevation.

 

Yet the 13 gorillas under Stanford's eye since 1996 are nothing like

Fossey's, which have often been likened to cows — slow-moving grazers of

poor nutritional vegetation like wild celery. At this lower elevation the

more active gorillas cover much more territory in a day and regularly

climb trees to gather fruit. (Stand clear when a 200-kilogram silverback

male is up a 50-metre tree, advises the researcher.)

 

Also roaming the same swath of forest and searching for food are 26

chimpanzees.

 

" The two species of apes give us a window into what the ecological

relationships might have been like for early humans living together, " says

Stanford.

 

From detailed feeding observations and by analyzing droppings, the

American and Ugandan research team has discovered that the ape diets are

more alike than anyone realized. Gorillas eat fruit whenever they can,

preferring a pineapple relative. The more agile chimps manage a fruit diet

throughout the year, with a heavy emphasis on figs. There's little

evidence of food fights but there is one big behavioural distinction.

 

" Meat eating is the pivotal ecological difference between chimps and

gorillas, " says Stanford.

 

Gorillas simply aren't interested in meat, even when a dead antelope is

lying right in their path. By contrast, chimps obviously crave meat, not

only to eat but to use in barter, swap for sex and withhold from their

rivals, say the researchers.

 

Their average daily meat intake of 65 grams may not sound like much in the

land of the quarter-pounder, but it's double earlier estimates.

 

This finding dovetails with a hypothesis posed last year by Stanford and

Caleb Finch, a colleague at the University of Southern California. They

suggested that around 2 1/2 million years ago, natural mutation introduced

a gene in some hominids that provided protection against the harmful

effects of cholesterol.

 

Humans have this gene. Chimps, the closest human relative, have a version.

Gorillas don't — to their cost. Zoos once fed eggs and meat to captive

gorillas out of misplaced nutritional zeal and the apes suffered a high

rate of heart disease.

 

So evolution did favour some meat in the modern human diet, just not so

much of it. The teeth in our mouths are a mess because that dietary

message didn't get translated fast enough anatomically, according to Peter

Lucas.

 

Lucas contends that the actual foods in the diet of early humans weren't

the crucial factor in determining how our dentition evolved. And there is

a lot of evolution, from as many as 50 teeth to no more than 32 now and

from broad sharp incisors and bulky molars to the even, perfect rows

flashed at the Oscars.

 

" It's what they did with the foods before they ate them that made the

difference, " says Lucas.

 

Consider cutting and cooking. A sharp stone edge meant that meat no longer

had to be torn in chunks but could instead be cut into small pieces. Ditto

for root vegetables.

 

Those canines and other front teeth quickly shrank, says Lucas.

 

Cooking made the big difference to molars and premolars. Lucas has

calculated that molars can be between 56 per cent and 82 per cent smaller

for eating a cooked potato rather than a raw one, depending on baked,

boiled, with skin or without.

 

Cooking meant teeth at the back of the mouth also got smaller but not as

quickly as those at the front and more slowly than the shrinking of the

jaw which wasn't working as hard. The result, says Lucas, is a mouth

usually lacking room for wisdom teeth and where the general disarray

(malocclusion to dentists) contributes to gum and periodontal disease.

 

As one member of the audience noted at the end of the session — to loud

groans — there's lots to chew on where the evolution of the human diet is

concerned.

 

Additional articles by Peter Calamai