|The McKay team states that the shapes of the objects in ALH 84001 could have been formed in any number of ways, from clay drying up or unusual crystallization of carbonate minerals. Edward Scott (University of Hawai'i) has drawn attention to the impact history of the rock and suggests that the unusual forms might have been produced by formation of the carbonates after an intense impact event. Perhaps the small tubular particles are the last to condense from a hot gas.|
J. William Schopf (University of California, Los Angeles), who has studied the oldest fossils on Earth, notes that the Martian objects are much smaller, by about a factor of 100, than are terrestrial microorganisms, living or fossil. Compare this image of a terrestrial microfossil with the image above. He also notes that proof of biological origin requires that cellular structures be identified, such as a cavity inside the tubular particles or the presence of cell walls. Nor is there any evidence that the objects are even partly composed of organic chemicals.