Page 146 - Introduction to Paleobiology and The Fossil Record

MACROEVOLUTION AND THE TREE OF LIFE 133

protein that carries oxygen in the blood, and and function of a protein. Small changes in
that makes the blood red. Structurally, the the amino acid sequence of hemoglobin
hemoglobin of all organisms that possess it is occurred every few million years, somewhat
very similar because it has to perform its at random, and the rate of change allows a
oxygen-carrying function – but there are time scale to be calibrated against the molecu-
subtle differences. So, the hemoglobin of lar tree.
humans and chimps is identical, but their Since 1990, attention has shifted almost
hemoglobin differs a little from that of a horse entirely from sequencing proteins to sequenc-
or cow, and a great deal from the hemoglobin ing the nucleic acids such as DNA and RNA.
of a shark or a salmon. These are the molecules in the nucleus that
Comparisons of molecules allow analysts comprise the genetic code, and they may be
to do two things: to draw up trees of relation- sequenced in a semiautomated manner using
ships and to estimate time. Trees of relation- a process called the polymerase chain reaction
ships can be based on a simple comparison of (PCR). PCR is a means of cloning, or dupli-
the amount of difference between protein cating, small samples of nucleic acid, and then
sequences, and a best-fi tting dendrogram, or of determining the exact sequence of base
branching diagram, is drawn. Identifying spe- pairs, the four components of the nucleic acid

cific amino acid changes, and treating them as strand, adenine, cytosine, guanine and thymine
synapomorphies, allows the dendrogram to (or uracil), abbreviated as A, C, G and T (or
be treated as a molecular cladogram. U). DNA and RNA may be sequenced from
Time estimation comes from the concept of the nucleus or the mitochondria of cells (see
the molecular clock. The amount of difference p. 186), and molecular biologists generate

in the fine structure of a protein between any huge sequences of such information each year.
pair of species is proportional to the time Indeed, the human genome project was one
since they last shared a common ancestor. of many examples of international programs
Differences have been documented in the to determine the entire DNA sequence of all
primary structure of proteins, the sequence of the chromosomes of a single species. The PCR
amino acids from end to end of the unfurled method has also opened up the possibility of
protein backbone. There are some 20 amino sequencing the genetic material of extinct
acids, and their sequence determines the shape organisms (Box 5.6).

Box 5.6 Fossil proteins: the real Jurassic Park?

Proteins were extracted from fossils in the 1960s and 1970s, but most of these were decay materials,
the proteins of bacteria that decomposed the original tissues. Even in cases of exceptional preserva-
tion where soft tissues are preserved (see p. 60), the proteins have usually long vanished. Until 1985,
the oldest DNA, recovered in tiny quantities, came from Egyptian mummies, 2400 years old.
Then came Jurassic Park! In the book by Michael Crichton (1990), and in the fi lm by Steven
Spielberg (1993), a scenario was developed where molecular biologists extracted dinosaur DNA from
blood retained in the stomach of a mosquito preserved in amber. The fragments of dinosaur DNA
were cloned and inserted into the living cells of a modern frog (an odd choice when the nearest
living relatives of dinosaurs are birds), and the whole dinosaur genetic code was somehow recon-
structed and living dinosaurs recreated. Amazingly, science then followed the fiction for a time.

Michael Crichton was wise to choose amber as the means of preservation (see p. 63). Insects in
amber are trapped instantly, usually overwhelmed by the sticky resin, and no decay takes place; the
amber excludes oxygen and water so that no physical or chemical changes should occur during

subsequent millennia. A series of scientific reports were published in high-profi le journals through
the 1990s, announcing original DNA from a termite in Oligocene-Miocene amber, a weevil in Early
Cretaceous amber, Miocene leaves, and even supposed dinosaur DNA in 1994. These reports col-

Continued