Der Mensch selbst war nie freiwillig Vegetarier. Lediglich der Australopithecus (Hominoide) hatte wohl noch einen sehr hohen Anteil an pflanzlicher Nahrung (war wahrscheinlich aber auch kein reiner Vegetarier). Später gab es eine stärkere Verschiebung hin zum "Allesfressertum" mit teilweise starker Fleischkomponente und damit auch zu gewissen Adaptionen in dies Richtung. Das wird durch ganz unterschiedliche wissenschaftliche Daten untermauert.
Und man glaubt es kaum: Sogar die Bandwürmer haben in letzter Zeit diesbezüglich noch mehr Licht ins Dunkel gebracht (danach haben unsere Vorfahren schon sehr lange(!) Fleisch gegessen)...

>Out of Africa: origins of the Taenia tapeworms in humans<
Wenn der Link (oder >der hier< ) nicht funzt, einfach mal im Archiv nach Hoberg: Out of Africa: origins of the Taenia tapeworms in humans. Proceedings: Biological Sciences, Volume 268, Number 1469 / April 22, 2001 suchen.
... oder deutscher Artikel: www.wissenschaft-online.de/artikel/759996

Nur noch zwei Zitate (leider nur englisch):

William R. Leonard:
All australopithecines had skeletal and dental features built for processing tough, low-quality plant foods. The later, robust australopithecines-- a dead-end branch of the human family tree that lived alongside members of our own genus-- had especially pronounced adaptations for grinding up fibrous plant foods, including massive, dish-shaped faces; heavily built mandibles; ridges, or sagittal crests, atop the skull for the attachment of powerful chewing muscles; and huge, thickly enameled molar teeth. (This is not to say that australopithecines never ate meat. They almost certainly did on occasion, just as chimps do today.) In contrast, early members of the genus Homo, which descended from the gracile australopithecines, had much smaller faces, more delicate jaws, smaller molars and no sagittal crests-- despite being far larger in terms of overall body size than their predecessors.
Together these features suggest that early Homo was consuming less plant material and more animal foods.
As to what prompted Homo's initial shift toward the higher-quality diet necessary for brain growth, environmental change appears to have once more set the stage for evolutionary change. The continued desiccation of the African landscape limited the amount and variety of edible plant foods available to hominids. Those on the line leading to the robust australopithecines coped with this problem morphologically, evolving anatomical specializations that enabled them to subsist on more widely available, difficult-to-chew foods. Homo took a different path. As it turns out, the spread of grasslands also led to an increase in the relative abundance of grazing mammals such as antelope and gazelle, creating opportunities for hominids capable of exploiting them. H. erectus did just that, developing the first hunting-and-gathering economy in which game animals became a significant part of the diet and resources were shared among members of the foraging groups. Signs of this behavioral revolution are visible in the archaeological record, which shows an increase in animal bones at hominid sites during this period, along with evidence that the beasts were butchered using stone tools.
(...)
The evolution of H. erectus in Africa 1.8 million years ago also marked a third turning point in human evolution: the initial movement of hominids out of Africa.
(...)
The impetus behind this newfound wanderlust again appears to be food. What an animal eats dictates to a large extent how much territory it needs to survive.
Carnivorous animals generally require far bigger home ranges than do herbivores of comparable size because they have fewer total calories available to them per unit area.
Large-bodied and increasingly dependent on animal foods, H. erectus most
likely needed much more turf than the smaller, more vegetarian australopithecines did. Using data on contemporary primates and human hunter-gatherers as a guide, Robertson, Susan C. Antón of Rutgers University and I have estimated that the larger body size of H. erectus, combined with a moderate increase in meat consumption, would have necessitated an eightfold to 10-fold increase in home range size compared with that of the late australopithecines-- enough, in fact, to account for the abrupt expansion of the species out of Africa.
Cordain et al.:
Recent isotopic evidence from the skeletons of Neanderthals (Richards et al, 2000) and Paleolithic humans (Richards & Hedges 2000) suggests that the dominance of animal foods in the human diet was not simply a recent phenomenon limited to contemporary hunter-gatherers, but rather has a long history in the human lineage. These studies provide objective data showing that the diets of hominids living in Europe during the Paleolithic were indistinguishable from top trophic level carnivores such as arctic foxes and wolves.
Indeed, hominids may have experienced a number of genetic adaptations to animal-based diets early on in our genus`s evolution analogous to those of obligate carnivores such as felines. Carnivorous diets reduce evolutionary selective pressures that act to maintain certain anatomical and physiological characteristics needed to process and metabolize high amounts of plant foods. In this regard, hominids, like felines, have experienced a reduction in gut size and metabolic activity along with a concurrent expansion of brain size and metabolic activity as they included more and more energy-dense animal food into their diets (Aiello & Wheeler, 1995; Cordain et al, 2001; Leonard & Robertson, 1994). Further, similar to obligate carnivores (Pawlosky et al, 1994), humans maintain an inefficient ability to chain elongate and desaturate 18 carbon fatty acids to their product 20 and 22 carbon fatty acids (Emken et al, 1992). Since 20 and 22 carbon fatty acids are essential cellular lipids, then evolutionary reductions in desaturase and elongase activity indicate that preformed dietary 20 and 22 carbon fatty acids (found only in animal foods) were increasingly incorporated in lieu of their endogenously synthesized counterparts derived from 18 carbon plant fatty acids. Finally, our species has a limited ability to synthesize taurine from precursor amino acids (Chesney et al, 1998; Sturman et al, 1975), and vegetarian diets in humans result in lowered plasma and urinary concentrations of taurine (Laidlaw et al, 1988). Like felines (Knopf et al, 1978; MacDonald et al, 1984), the need to endogenously synthesize taurine may have been evolutionarily reduced in humans because exogenous dietary sources of preformed taurine (found only in animal food), had relaxed the selective pressure formerly requiring the need to synthesize this semi-conditional amino acid.