Lecture 15: The Origins of Life
Third, we must explain the creation of the exquisitely organized billion-
atom molecules of DNA (deoxyribonucleic acid), the “software” that
controls reproduction. This is the toughest part of the puzzle. Without DNA,
reproduction was inaccurate, and “chemical evolution” would have been
slow and unreliable. DNA consists of two linked chains of nucleotides, linked
by bonds like rungs on a ladder. Each bond consists of two “bases” (small
clusters of atoms) that come in only four types and can only ¿ t together in
certain ways. They are known as Adenine, Thymine, Cytosine, and Guanine.
A (Adenine) links only with T (Thymine), and C only with G. So, along each
chain, you have a sequence of the four bases, each linked to its complement
on the other chain. The exact sequence of these bonds codes the information
used to construct each organism. When DNA reproduces, the bonds split in
two and the two chains separate. The bases seek out their complements from
the chemicals surrounding them (A looks for T and so on), and in this way
two new chains of DNA appear, identical to the originals.This mechanism is the key to accurate reproduction. But there’s a problem.
DNA cannot exist on its own, yet cells cannot survive without DNA—so
which evolved ¿ rst, the software of DNA or the hardware of the cell? At
present, the best bet is that RNA (ribonucleic acid), a single-stranded variant
of DNA, may have acted as both “hardware” and “software.” Because it
is similar to DNA, RNA can code for genetic information, but it can also
act like an enzyme and help manufacture the molecules that cells need.
However it was done, life seems to have appeared quickly on Earth, for
living organisms existed by 3.8 billion years ago. This suggests that life may
appear everywhere in the Universe that conditions are right. Though we
don’t yet know all the details, we understand enough to know that life can be
assembled from nonliving ingredients. ŶChristian, Maps of Time, chap. 4.
Delsemme, Our Cosmic Origins, chap. 5.Essential Reading