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March 20, 2006
Could We Tell Life If We Saw It?
ALH84001 in 2004
Joe Kirschvink
California Institute of Technology
34 min. (slideshow requires QCShow Player)
Audio only (mp3 format)
View as a webpage (quicktime, real player) (notes)
How can we hope to distinguish true biological microfossils from random assemblages of crystalline mineral material — especially if the life that those microfossils might represent were potentially an independent origin of life, billions of years ago, on another planet, and is now likely extinct? That's the question that has raged around the structures found in the Allen Hills 84001 meteorite.
As UCLA's William Schopf has written, "There are fine lines between what is known, guessed, and hoped for, and because science is done by real people these lines are sometimes crossed. But science is not a guessing game. The goal is to know. 'Possibly... perhaps... maybe' are not firm answers and feel-good solutions do not count. With regard to the famed Mars meteorite, for example, life either once existed on Mars or it didn't. Meteorite ALH84001 either holds telling evidence or it doesn't. Eventually, hard facts will sort it out."
Schopf has been among the harshest critics of the earliest interpretations of life in ALH84001. "Probably the best way to avoid being fooled by nonbiologic structures is to accept as bona fide fossils only those of fairly complex form. This may seem an unreasonably stringent rule for truly ancient fossils since the earliest kinds of cellular life (here and presumably elsewhere) almost certainly were very simple — probably individual, tiny, spheroidal cells. But until we have a sounder base of knowledge and better rules to separate nonfossils from true, it is best to err on the side of caution."
CalTech's Joe Kirschvink agrees, but comes to a different conclusion. Microfossil paleontologies based on morphologies are undoubtedly flawed. In morphology's place however, Kirschvink compellingly argues that the fingerprint of natural selection can be detected by the very complexity and purity of the results that selection produces. While there are no biological processes that can not be reproduced in some manner by non-life processes, the results of simple inorganic syntheses are haphazard at best.
Magnetite exists in ALH84001, and Kirschvink argues that it was biologically produced, primarily by subjecting it to a Venn diagram analysis of seven different physical characteristics, each ranging from hard to easy, and in the process pointing out that ALH84011's magnetite is of an even higher quality than is capable of currently being manufactured by human processes.
Magnetotactic bacteria were discovered on Earth only in the 1960's, but we now know of south- and north-pole seeking bacteria. For an organism evolved to exist in ponds within a narrow range of oxic-anoxic conditions, where light doesn't penetrate and gravity is overwhelmed by random Brownian motions, the evolution of magnetotaxis is an exceptionally clever solution to the problem determining orientation.
Although Mars no longer has either a magnetic field or liquid water, it is strongly presumed that Mars once had both, and the most parsimonious, simplest explanation for the high-quality magnetite crystals that appear in ALH84001 is that they were synthesized by organisms similar to terrestrial bacteria.
— Wirt Atmar
About the Speaker
Joe Kirschvink is widely known as an original thinker and an excellent teacher. Among his major contributions are:
the idea that biogenic magnetite produced by the magnetotactic bacteria ("magnetofossils") might be responsible for the magnetization of some sedimentary rocks; these magnetofossils now provide the strongest evidence for early life on the planet Mars.
A second idea was that the magnetic field sensitivity in animals might be due to small chains of the same biogenic magnetite functioning as specialized sensory organelles; this work has provided a solid biophysical basis for understanding magnetic effects on animal behavior, and has actually led to the discovery of these new sensory organs in higher animals.
An idea that is generating much interest recently is that the entire Earth may have actually frozen over several times in Earth history, resembling a "Snowball", potentially causing some of the most severe crises in the history of life on Earth and perhaps stimulating evolution.
Another original concept is that the Cambrian Evolutionary explosion may have been precipitated in part by large burst of true polar wander, in which the Earth's rotational axis moved to the equator in a geologically short interval to of time, and...
that the burst of biomineralization observed in the fossil record at the Cambrian Explosion may have resulted from the evolutionary exaptation of the magnetite biomineralization system.
A common thread in all of these efforts is the study of paleomagnetism and rock magnetism, for which the Kirschvink group maintains laboratories dedicated to the study of weakly magnetic biological and geological materials.
Joe recently won the Richard P. Feynman Prize for teaching excellence at Caltech.
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