Sunday, October 23, 2005

Spirochete Genes are Everywhere!

Edge's John Brockman likes to ask provocative questions. At the beginning of this year, he asked "What do you believe is true even though you cannot prove it?" He got many interesting answers from many interesting people, but two were good enough that I edited them down to be short enough to use in my signature line. The precis for Lynn Margulis' answer looked like this in my signature:

All sensory cells [in all animals] have in common the presence of
... cilia [with a constant] structure.  It provides a strong
argument for common ancestry.  The common ancestor ... was a
spirochete bacterium. 
  --Lynn Margulis ( 

The blog gives me the opportunity to quote the whole thing where a few more people might read it. Margulis wrote:

Our ability to perceive signals in the environment evolved directly from our bacterial ancestors. That is, we, like all other mammals including our apish brothers detect odors, distinguish tastes, hear bird song and drum beats and we too feel the vibrations of the drums. With our eyes closed we detect the light of the rising sun. These abilities to sense our surroundings are a heritage that preceded the evolution of all primates, all vertebrate animals, indeed all animals. Such sensitivities to wafting plant scents, tasty salted mixtures, police cruiser sirens, loving touches and star light register because of our "sensory cells".

These avant guard cells of the nasal passages, the taste buds, the inner ear, the touch receptors in the skin and the retinal rods and cones all have in common the presence at their tips of projections ("cell processes") called cilia. Cilia have a recognizable fine structure. With a very high power ("electron") microscope a precise array of protein tubules, nine, exactly nine pairs of tubules are arranged in a circular array and two singlet tubules are in the center of this array. All sensory cells have this common feature whether in the light-sensitive retina of the eye or the balance-sensitive semicircular canals of the inner ear. Cross-section slices of the tails of human, mouse and even insect (fruit-fly) sperm all share this same instantly recognizable structure too. Why this peculiar pattern? No one knows for sure but it provides the evolutionist with a strong argument for common ancestry. The size (diameter) of the circle (0.25 micrometers) and of the constituent tubules (0.024 micrometers) aligned in the circle is identical in the touch receptors of the human finger and the taste buds of the elephant.

What do I feel that I know, what Oscar Wilde said (that "even true things can be proved")?

Not only that the sensory cilia derive from these exact 9-fold symmetrical structures in protists such as the "waving feet" of the paramecium or the tail of the vaginal-itch protist called Trichomonas vaginalis. Indeed, all biologists agree with the claim that sperm tails and all these forms of sensory cilia share a common ancestry.

But I go much farther. I think the the common ancestor of the cilium, but not the rest of the cell, was a free-swimming entity, a skinny snake-like bacterium that, 1500 million years ago squiggled through muds in a frantic search for food. Attracted by some smells and repelled by others the bacteria, by themselves, already enjoyed a repertoire of sensory abilities that remain with their descendants to this day. In fact, this bacterial ancestor of the cilium never went extinct, rather some of its descendants are uncomfortably close to us today. This hypothetical bacterium, ancestor to all the cilia, was no ordinary rod-shaped little dot.

No, this bacterium who still has many live relatives, entered into symbiotic partnerships with other very different kinds of bacteria. Together this two component partnership swam and stuck together both persisted. What kind of bacterium became an attached symbiont that impelled its partner forward? None other than a squirming spirochete bacterium.

The spirochete group of bacteria includes many harmless mud-dwellers but it also contains a few scary freaks: the treponeme of syphilis and the borrelias of Lyme disease. We animals got our exquisite ability to sense our surroundings—to tell light from dark, noise from silence, motion from stillness and fresh water from brackish brine—from a kind of bacterium whose relatives we despise. Cilia were once free-agents but they became an integral part of all animal cells. Even though the concept that cilia evolved from spirochetes has not been proved I think it is true. Not only is it true but, given the powerfulnew techniques of molecular biology I think the hypothesis will be conclusively proved. In the not-too-distant future people will wonder why so many scientists were so against my idea for so long!

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