"Co-evolution" is a term evolutionary theorists use to describe ecological intimacy, when the evolution of one organism is shaped by and in turn shapes the evolution of another (Ehrlich and others 1988). It is interesting to consider the evolutionary trajectories of desert biota and of human beings in that light. Relative to the time-scales of the geologic and atmospheric processes that created desert conditions, the advent of humans is a very recent event. Nonetheless, the effect of humans on deserts - and of deserts on humans - is pronounced.

Desert landscapes and desert biota have had profound effects on human cultural evolution. Humans display remarkable behavioral and cultural adaptations to the aridity and unpredictability of deserts, and traditions derived there have influenced human and biological communities far beyond the desert edge; that the three "religions of the book" had their origins in these environments well-illustrates that fact (see Chapter ). Plants and animals from these harsh landscapes have also played an important role in the evolution of modern human societies. Dryland biota provided much of the "raw material" for species that could and did become domesticated, which helped usher in the dawn of pastoral and agricultural societies. The early domestication of ungulates (cattle, sheep, and goats) began in the drylands of West Asia, on the edge of the Arabian Deserts, some 9 000 years ago (Davis 005), and the domestication of llamas and alpacas took place in the Andean Puna of South America some 6 000 years ago just north of South America's "arid diagonal" formed by the Atacama, Dry Puna, and Monte deserts (Table 1.3). In many regions of the world, dryland annuals have been at the base of the plant domestication process and drylands have been the cradle of agricultural societies. The first records of cultivated wheat and barley (two dryland ephemerals) come from the Fertile Crescent of West Asia some 7-9 000 years ago. In the American Continent the first agricultural records come from the Tehuacán Valley in southern Mexico, a hot tropical dryland where corn and squash (two annual, droughttolerant fast growers) were first domesticated some 6 000 years ago. Not too long after that, gatherers in the Andean Puna started domesticating two other dryland ephemerals: the quinoa (Chenopodium, a fast-growing annual) and the potato (Solanum, a tuber ephemeral).

That biota of drylands would be a source of such innovation is not surprising, given the life history of those plants and animals. Arid-land herbivores, in particular desert ungulates, are extremely hardy. They can use water very efficiently, they can withstand long periods without drinking, and when forage is plentiful they can quickly convert plant material into animal protein with very high efficiency. Furthermore, many of them are migratory and move naturally in herds following a leader, looking for new foraging grounds, and socially protecting themselves from predators. For all these evolutionary reasons, ungulates native to drylands were ideal candidates for domestication: hardy animals, efficient foragers, and amenable to shepherding, as social aggregation is a natural behaviour for them. Some of the same factors that made wild goats, mountain sheep, or guanacos evolutionarily adapted to desert environments are what drove early hunter-gatherers to start breeding their offspring and selecting them for desirable domestic attributes. As with desert ungulates, the same traits that have made some desert annuals apt to survive and thrive on ephemeral water pulses are what make them so apt for agriculture: fast growth, short life cycle, and the capacity to direct most of their metabolic budget towards the abundant production of seeds. Because dryland ephemerals grow so fast and produce so much seed in just a few weeks, they grow at an amazingly fast rate when planted at the desert's edge and make ideal grain plants, especially cereals and pulses.

The effect of humans on the ecology and the evolutionary trajectory of deserts can be similarly pronounced. The following ecological "anachronism" provides an illustrative example: some desert plants have seed dispersal mechanisms that reflect the existence of seed dispersers that are no longer present. Trees like the mesquites (Prosopis), for example, have pods with nutritious sweep pulp and extremely tough seeds which need intense scarification in order to germinate. Similarly, the tough seeds of the prickly pears (Platyopuntia) germinate successfully only when chewed and digested for a long time. During the Pleistocene period, this abrasion was provided by the digestive system of large ungulates, such as gomphotheres or giant ground sloths. At the end of the last glaciation some 15 000 years ago, however, much of that Pleistocene megafuana went extinct - a fate that humans likely contributed to (Alroy 001, Brook and Bowman 004). Loss of that fauna resulted in the loss of seed dispersal and regeneration mechanisms for a number of plant species. Desert plant species with anachronic seed dispersal have merely survived for the last millennia through vegetative growth and accidental abrasion of seeds in the deserts' sand and gravel, in the absence of their effective seed dispersers. Not surprisingly, when humans reintroduced ungulates - cattle - into the New World some five centuries ago, the population of many of these plant species rebounded to large numbers.

Humans continue to affect desert ecology, at times fundamentally. Being areas of such low productivity, deserts can be easily degraded - even irreparably - by the increasing intensity of human land and resource use. Desert soils, which are of generally limited profundity and high fragility (see Box 1.1), are highly susceptible to compaction, erosion, and salinization when exploited for agricultural, industrial, or recreational purposes. Invasive nonnative plants, whether introduced intentionally (such as in the case of the planting of grasses for livestock forage, which have the effect of introducing a grass-fire cycle to an ecosystem that has no natural fire regime) or not (such as the case of the invasion of Tamarix ramosissima in Nearctic deserts, which can substantially alter desert hydrological regimes), can have cascading effects on ecosystem function and native species viability in deserts. Human industry in and beyond deserts alters not only desert weather patterns via anthropogenic climate change, but also desert nutrient cycling via atmospheric deposition. Paradoxically, fertilization of deserts through increased deposition of nutrients like nitrogen can favour the invasive dispersal of non-native species and reduce native diversity. Whether through direct or indirect pathways, humans clearly have a hand in determining the future course of desert evolution.