Answer:
The last permanent high-elevation ice masses were rapidly melting 14,000 years ago, and the regional climate was becoming drier and modern (interglacial) by 12,000 years ago. This last climate change marks the birth of the modern Sonoran Desert ecosystem.
Step-by-step explanation:
The most recent expansion of the Sonoran Desert into its modern area in Arizona and California occurred only 9000 years ago, with the modern cThe Sonoran Desert that we see today, with its characteristic assemblages of plants and animals is quite recent, at least in terms of geologic time. In fact, it and the other North American deserts are among the youngest biotic communities on the continent. Although some Sonoran species evolved in ancestral seasonally-dry tropical communities, the development of the unique regional climates and the evolution of characteristic desert-adapted plants and animals are thought to have combined to form the Sonoran Desert by about 8 million years ago (mya) in the late Miocene. Similar conditions developed many times subsequently as global climates changed, with the Sonoran Desert continually expanding, contracting, and redefining itself. The most recent expansion of the Sonoran Desert into its modern area in Arizona and California occurred only 9000 years ago, with the modern communities of plants and animals developing 4500 years later. This chapter is a walk through time examining the conditions that led to the development of the Sonoran Desert and exploring what shaped its dynamic historyommunities of plants and animals developing 4500 years later.