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In the new work, the scientists used carbon isotope analysis to extract the dietary signal from fossilized teeth of African hominin species dating from 4.5-1.5 million years ago.

Isotope analysis is based on the concept that “you are what you eat.” The carbon isotopic composition of past meals is incorporated into the forming tooth enamel, frozen in time, yet recoverable after millions of years. Carbon isotopes are especially valuable for distinguishing diets based on “C3” or “C4” plants, which differ in how efficiently carbon is recovered during photosynthesis. C3 plants include trees, shrubs and herbs, and predominate in forested conditions or around well-watered areas. C4 plants are the grasses and sedges that inhabit primarily open environments. In living animals, carbon isotope ratios in tooth enamel readily distinguish grass feeders, like zebras and wildebeests, from fruit or leaf eaters, like chimpanzees and giraffes.

The diets of the earliest hominin species, Ardipithecus ramidus and Australopithecus anamensis from 4.4 to 4 million years ago, were dominated by C3 plants. But beginning around 3.5 million years ago, data from Hadar and Dikika in Ethiopia and West Turkana in Kenya show that A. afarensis and another species called Kenyanthropus platyops consumed foods from both C4 and C3 plant communities. This expansion in dietary preference represents the first use of C4 plant foods that had been abundant in the environment for at least one million years. All subsequent hominin species were capable of consuming both C3 and C4 foods, a flexible adaptation that was inherited by modern humans.

Lucy's species, Australopithecus afarensis, were thought to be "C3" plant eaters, which includes trees, nuts and shrubs, but new analysis shows they may have been the first hominins to include grasses and sedges in their diets – a flexible adaptation that was inherited by modern humans.

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Did hominids eat fruits and veggies during the Neanderthal era?

They definitely ate fruit. Last year, paleoanthropologists found bits of date stuck in the teeth of a 40,000-year-old Neanderthal. There's evidence that several of the fruits we enjoy eating today have been around for millennia in much the same form. For example, archaeologists have uncovered evidence of 780,000-year-old figs at a site in Northern Israel, as well as olives, plums, and pears from the paleolithic era. Researchers have also dug up grapes that appear to be 7 million years old in northeastern Tennessee (although, oddly, the grapes are morphologically more similar to today’s Asian varieties than the modern grapes considered native to North America). Apple trees blanketed Kazakhstan 30,000 years ago, oranges were common in China, and wild berries grew in Europe. None of these fruits were identical to the modern varieties, but they would have been perfectly edible.