Let me show you how this works in the following example. Archaeologists surveyed an area north of Phoenix. They found a series of rock alignments and piles. They observed a particular type of agave plant growing in several spots.

What we know (through observation) is that this type of agave does not grow naturally in this region of Arizona. We also know that it reproduces in a certain way. The plant first sends out a tall stalk that has blooms at the end. These are fertilized by bats and insects. Instead of growing seeds, this plant actually produced small plants at the end of that stalk. The stalk eventually deteriorates and the plants land on the soil to hopefully grow roots and mature.

What happens in regions such as this one is that it is hotter than where the plant naturally grows. Therefore, the plant does not reproduce effectively in this environment. Instead, it relies on a backup system of reproduction sending out roots that spread new plants. This is a less than effective backup, but it has left a few plants for the archaeologists to find.

What else we know is that agave, particularly this type of agave, has many uses. It can be roasted in an oven and cooked. It is high in nutrient values. We know that living people who use agave have used it for other purposes included for medicine and even clothing. So it is a versatile plant with many uses. We also know that in other areas, the prehistoric people of southern Arizona grew this plant in large fields where they constructed rock alignments and rock piles. Through experiments, we have discovered that the moisture around the base of the rock piles increases thereby providing a little extra moisture to plants that are growing at the base of these piles. The rock alignments tend to trap water as it runs across the ground thereby also enhancing a region so that plants can capture more of the ground moisture. In a hot, dry desert environment, rock alignments and rock piles are an ingenious way to enhance growth of plants such as agave.

So what can we infer about all of this? Well, we can infer that people brought this plant into this region from where it naturally grew. That they probably had to help the plant reproduce. Since the plant puts off more than 50 little plants per stalk, this means they could use a great number of agave plants for food and other things and keep a ready supply of plants growing through their intervention. They simply had to break the stalk and plant the new "seedlings" in the right areas - such as at the base of pile of rocks, which increase soil moisture stimulating plant growth. We can, therefore, infer that the agave growing in this area was part of an elaborate agricultural system operating prehistorically.

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