Greetings Ready to Debate Evolution!

Genetic and structural analysis

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Originally posted by Morat
  Excellent. I recommend you start with, say..the relevant journals, where debate is ongoing right now as to the earliest forms of life.

    Do you have access to the major journals for evolutionary biology and biochemistry?

 Here is a nice list, and you might check here as well.

 

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William Schopf, who was present at the NASA conference, voiced many of the objections listed above (see Grady et al., 1996). “The biological explanation,” he said, “was unlikely.” Another critic was John F. Kerridge—lead author on a paper advising NASA on a strategy for finding life on Mars. Although impressed with the science in the paper, Kerridge concluded that it “fell far short of establishing the case for evidence of biological activity.” Even McKay, as the project’s team leader, denied having found the “smoking gun” of life, let alone “absolute proof_” of life, either past or present.

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Maybe we are alone after all
Recall that the meteorite in question originated on Mars, then was blasted into space by an asteroid collision. It was found in Antarctica in 1984 after laying around for some 12,000 years.

After inspecting part of the meteorite, NASA's David McKay and colleagues located tiny carbonate structures and organic molecules on the meteorite that seemed to have been formed by bacteria on Mars more than 1.3 billion years ago.

fossil

Magnification of "fossils" in the Martian meteorite and the "red planet" (left) courtesy of NASA.

A sample of the Mars rock to scale against a paper clip. © UW-Madison/John Valley.

A mars meteorite sample clearly showing the orange-yellow carbonate globules. © UW-Madison/John Valley (The Red Planet is now frozen and dry, but it was much warmer and wetter back then.)

Although the assertion was controversial from the first, only now have scientists analyzed the meteorite's main organic compounds. The results are fueling the doubters. "This is bad news with respect to using these meteorites to assess whether there ever was or is life on Mars," says Jeffrey Bada. Bada, director of the NASA Specialized Center of Research and Training in Exobiology at the Scripps Institution of Oceanography in California, was the main author of one of the new reports.

While not eliminating the possibility that life once existed on Mars, the new findings do seem to nullify the evidence for it.

Oops!
Bada's team spent years looking at amino acids in fragments from various meteorites. In August, 1997, they received fragments of the potato-sized meteorite that caused the sensation in 1996. okay, it's really smallThe researchers studied amino acids because they are the building blocks for proteins and enzymes and thus are essential to all known forms of life.

Amino acids and some other molecules can form in either of two mirror images. Like your own hands, both versions of these so-called "handed" chemicals look alike. But just as your left hand won't fit a right-hand glove, the opposite versions of handed chemicals are not identical. This distinction matters: biological systems only work if the handed chemicals are all in the same form. (On Earth, all amino acids used to make proteins are left-handed.)

Normally, you'd expect that the discovery of amino acids -- a foundation of life -- in a rock from Mars would be exciting news. But Bada's analysis showed that almost all of those amino acids were lefties -- just like life on Earth and other amino acids in the ice where the meteorite lay for 12,000 years.

More telling was the particular amino acids found in the meteorite. While earthly life uses more than 20 of these building blocks, only four were found in the meteorite -- exactly the same cast of characters as found in the ice. To Bada, the conclusion was inescapable. "They are clearly terrestrial and they look similar to amino acids we see in the surrounding Antarctic ice."

Believe it or not, scientists usually choose simple explanations over complex ones. And Bada says the simplest explanation for his findings was that the organic chemical had originated on Earth. And the second new study only reinforced that line of thought.

Another shoe drops
To understand the second study, we must return to McKay's 1996 report, which speculated that microbes had formed peculiar mineral grains found inside carbonate structures in the meteorite. As further evidence of life, McKay pointed to organic molecules in the meteorite.

I'm ready for my closeup But the study by A.J. Timothy Jull of the University of Arizona, also reported Jan. 16, undercuts that notion. Rather than inspect amino acids, Jull looked at the concentrations of two isotopes of carbon, the most important element in living things. (Isotopes are types of elements containing different numbers of neutrons; an isotope's number denotes its atomic weight.)

Since the carbonates and the organic material were supposedly formed by microbes that were taking in the same chemicals from the Martian environment, Jull reasoned that both should have contained the same proportions of carbon-13 and carbon-14. But he found "the organic material contains carbon-14 and the carbonate doesn't."

Carbon-14 often forms in Earth's upper atmosphere when an energetic particle like a cosmic ray blasts into a nitrogen nucleus, ejecting a proton. Thus Jull concluded that the carbonate minerals, which lack carbon-14, must have come from "somewhere in space, presumably Mars, and the organic material is a recent addition which took place while the meteorite was sitting on the ice." The disparity between the isotopes proved "there is no connection between the two things."

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