A meteor stream is composed of dust particles that have been ejected from a parent comet at a variety of velocities. These particles follow the same orbit as the parent comet, but due to their differing velocities they slowly gain on or fall behind the disintegrating comet until a shroud of dust surrounds the entire cometary orbit. Astronomers have hypothesized that a meteor stream should broaden with time as the dust particles' individual orbits are perturbed by planetary gravitational fields. A recent computer-modeling experiment tested this hypothesis by tracking the influence of planetary gravitation over a projected 5,000-year period on the positions of a group of hypothetical dust particles. In the model, the particles were randomly distributed throughout a computer simulation of the orbit of an actual meteor stream, the Gemini. The researcher found, as expected, that the computer-model stream broadened with time. Conventional theories, however, predicted that the distribution of particles would be increasingly dense toward the center of a meteor stream. Surprisingly, the computer-model meteor stream gradually came to resemble a thick-walled, hollow pipe.
Whenever the Earth passes through a meteor stream, a meteor shower occurs. Moving at a little over 1,500,000 miles per day around its orbit, the Earth would take, on average, just over a day to cross the hollow, computer-model Geminid stream if the stream were 5,000 years old. Two brief periods of peak meteor activity during the shower would be observed, one as the Earth entered the thick-walled "pipe" and one as it exited. There is no reason why the Earth should always pass through the stream's exact center, so the time interval between the two bursts of activity would vary from one year to the next.
Has the predicted twin-peaked activity been observed for the actual yearly Geminid meteor shower? The Geminid data between 1970 and 1979 show just such a bifurcation, a secondary burst of meteor activity being dearly visible at an average of 19 hours (1,200,000 miles) after the first burst. The time intervals between the bursts suggest the actual Geminid stream is about 3,000 years old.

Line	Micro-wear patterns found on the teeth of long-
	extinct specimens of the primate species
	australopithecine may provide evidence about their
	diets. For example, on the basis of tooth micro-wear
(5)	patterns, Walker dismisses Jolly’s hypothesis that
	australopithecines ate hard seeds. He also disputes
	Szalay’s suggestion that the heavy enamel of
	australopithecine teeth is an adaptation to bone
	crunching, since both seed cracking and bone
(10)	crunching produce distinctive micro-wear
	characteristics on teeth. His conclusion that
	australopithecines were frugivores (fruit eaters) is
	based upon his observation that the tooth micro-
	wear characteristics of east African
(15)	australopithecine specimens are indistinguishable
	from those of chimpanzees and orangutans, which
	are commonly assumed to be frugivorous primates.
	However, research on the diets of
	contemporary primates suggests that micro-wear
(20)	studies may have limited utility in determining the
	foods that are actually eaten. For example, insect
	eating, which can cause distinct micro-wear
	patterns, would not cause much tooth abrasion in
	modern baboons, who eat only soft-bodied insects
(25)	rather than hard-bodied insects. In addition, the
	diets of current omnivorous primates vary
	considerably depending on the environments that
	different groups within a primate species inhabit; if
	australopithecines were omnivores too, we might
(30)	expect to find considerable population variation in
	their tooth micro-wear patterns. Thus, Walker’s
	description of possible australopithecine diets may
	need to be expanded to include a much more
	diverse diet.