Ancient biomolecules and tissues

I cannot finish this chapter without mentioning the Jurassic Park
scenario: discovering dinosaur DNA, using modern biotechnology
to reconstitute that DNA, and using this to bring the dinosaur
back to life.

There have been sporadic reports of finding fragments of dinosaur
DNA in the scientific literature over the past decade, and then using
PCR (polymerase chain reaction) biotechnology to amplify the
fragments so that they can be studied more easily. Unfortunately,
for those who wish to believe in the Hollywood-style scenario,
absolutely none of these reports have been verified, and in truth it is
exceedingly unlikely that any genuine dinosaur DNA will ever be
isolated from dinosaur bone. It is simply the case that DNA is a long
and complex biomolecule which degrades over time in the absence
of the metabolic machinery that will maintain and repair it, as
occurs in living cells. The chances of any such material surviving
unaltered for over 65 million years while it is buried in the ground
(and subject there to all the contamination risks presented by
micro-organisms and other biological and chemical sources, and
ground water) are effectively zero.

All reports of dino-DNA to date have proved to be records of
contaminants. In fact the only reliable fossil DNA that has been
identified is far more recent, and even these discoveries have
been made possible because of unusual preservational conditions.
For example, brown bear fossils whose remains are dated back to
about 60,000 years have yielded short strings of mitochondrial
DNA – but these fossils had been frozen in permafrost since the
animals died, providing the best chance of reducing the rate of
degradation of these molecules. Dinosaur remains are of course
1,000 times more ancient than those of arctic brown bears.
Although it might be possible to identify some dinosaur-like genes
in the DNA of living birds, regenerating a dinosaur is beyond the
bounds of science.

One final, but extremely interesting, set of observations concerns
the analysis of the appearance and chemical composition of the
interior of some tyrannosaur bones from Montana. Mary Schweitzer
and colleagues from North Carolina State University were given
access to some remarkably well preserved T. rex bones collected by
Jack Horner (the real-life model for ‘Dr Alan Grant’ in the film
Jurassic Park). Detailed examination of the skeletal remains
suggested that there had been minimal alteration of the internal
structure of the long bones; indeed, so unaltered were they that the
individual bones of the tyrannosaur had a density that was consistent
with that of modern bones that had simply been left to dry.
Schweitzer was looking for ancient biomolecules, or at least the
remnant chemical signals that they might have left behind. Having
extracted material from the interior of the bones, this was powdered
and subjected to a broad range of physical, chemical, and biological
analyses. The idea behind this approach was not only to have the
best chance of ‘catching’ some trace, but also to have a range of
semi-independent support for the signal, if it emerged. The burden
really is upon the researcher to find some positive proof of the
presence of such biomolecules; the time elapsed since death and
burial, and the overwhelming probability that any remnant of such
molecules has been completely destroyed or flushed away, seem to
be overwhelming. Nuclear magnetic resonance and electron spin
resonance revealed the presence of molecular residues resembling
haemoglobin (the primary chemical constituent of red blood cells);
spectroscopic analysis and HPLC (high performance liquid
chromatography) generated data that was also consistent with the
presence of remnants of the haeme structure. Finally, the dinosaur
bone tissues were flushed with solvents to extract any remaining
protein fragments; this extract was then injected into laboratory
rats to see if it would raise an immune response – and it did! The
antiserum created by the rats reacted positively with purified avian
and mammalian haemoglobins. From this set of analyses, it seems
very probable that chemical remnants of dinosaurian haemoglobin
compounds were preserved in these T. rex tissues.

Even more tantalizingly, when thin sections of portions of bone
were examined microscopically, small, rounded microstructures
could be identified in the vascular channels (blood vessels) within
the bone. These microstructures were analysed and found to be
notably iron-rich compared to the surrounding tissues (iron being
a principal constituent of the haeme molecule). Also the size and
general appearance was remarkably reminiscent of avian nucleated
blood cells. Although these structures are not actual blood cells,
they certainly seem to be the chemically altered ‘ghosts’ of the
originals. Quite how these structures have survived in this state
for 65 Ma is a considerable puzzle.

Schweitzer and her co-workers have also been able to identify
(using immunological techniques similar to the one mentioned
above) biomolecular remnants of the ‘tough’ proteins known as
collagen (a major constituent of natural bone, as well as ligaments
and tendons) and keratin (the material that forms scales, feathers,
hair, and claws).

Although these results have been treated with considerable
scepticism by the research community at large – and rightly so, for
the reasons elaborated above – nevertheless, the range of scientific
methodologies employed to support their conclusions, and the
exemplary caution with which these observations were announced,
represent a model of clarity and application of scientific
methodologies in this field of palaeobiology.