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remains convinced that the particles of magnetite that were found in the sample once constituted a part of a Martian life form. Magnetite is found in various places on our planet, but one of the most interesting homes for this magnetic mineral is inside single-celled organisms that employ a unique style of navigation. So-called magnetotaxic animals use particles of magnetite as tiny compasses that orient their bodies with planetary geography. Though these magnetite bodies take advantage of the earth’s magnetic field in exactly the same way that makes the Boy Scout compass face north, in this case it is not to help them to read maps correctly but to do something much simpler: the magnetite pulls these tiny aquatic animals downward into the lakebeds lining their watery homes, where they find food, safety, and comfortable temperatures. The origin of the magnetite found in McKay’s samples is a matter that still swirls in controversy, but if he is correct, not only will his discovery constitute the first evidence of extraterrestrial life but his claim will be based on an elementary form of navigation.
    The rudimentary navigational tools that I have described are based on a mechanism that allows an animal to drift up or down a gradient of light, heat, magnetism, or the concentration of some chemical. Such mechanisms can serve a variety of functions where animals need to get from where they are to an easily defined target such as a strong source of light or a warm pool of water. Simpleas they are, some things are still not well understood about these elementary mechanisms. Indeed, some of the fine details of bacterial navigation have led researchers to suggest that these tiny beings possess a type of cognition not different in kind from that found in much larger multicellular animals.
    When a hungry urban primate tries to zero in on Sarah’s Spicy Potatoes in the buffet line, this is yet another form of taxis, but for reasons that will soon be clear, the technical hurdles that must be overcome to reach such targets are considerably more complicated than those faced by the average amoeba or slime mold.
THE POWER OF TWO
    A frog sits motionless at the edge of a muddy stream, seemingly oblivious to the passage of time and the flow of events. When a fly happens within striking range, the frog’s tongue lashes out to capture it with such speed and precision that the fly seems to have vanished into thin air. Clever scientific experiments using time-lapse photography have shown that the frog can not only discern the direction of the fly’s movement but also assess the fly’s distance with enough precision to ensure accurate contact between sticky tongue tip and hapless fly torso. 3
    Though prey catching in frogs might seem very different from taxic behavior in bacteria, what they share is that they are simple behaviors designed to help an animal make a connection with a spatial target. One advantage that an animal like a frog has over a microscopic one-celled critter is simply that of size. With a big enough body, sensors can be placed in such a way that they can be used to triangulate on the location of a target. A pair of sensors—the eyes in this case—can make precise estimates of the locations of target objects without having to engage in the complicated trial-and-error methods used by much smaller animals.
    Bilateral symmetry (that is, a body composed of two more or less identical halves) is common in nature, and with such symmetry comes paired sense organs. The mechanism by which pairs of sensors can produce useful orienting behaviors can be exceedingly simple. A basement hobbyist can easily construct a small machine capable of such seeking behaviors using nothing more than a pair of sensors (for example, simple light detectors that can be purchased for a few pennies at an electronics shop), a pair of wheels, and a powered motor. By wiring the machine together in such a way that each sensor is attached to a wheel on the