Human cells may contain clues about the origin of life because: 1) While human cells are complex, they still retain molecular remnants from early life under layers of complexity. 2) Bacteria appear to have lost RNA and other molecular remnants of early life through simplification over time. 3) We know more about human cell biochemistry than simple bacteria, so human cells may harbor more insights into early life origins. The document discusses how human cells, as eukaryotes, may provide insights into the origins of life despite their complexity. It notes that bacteria appear to have simplified over time, losing molecular remnants of early RNA-based life. However, as we understand human cell biochemistry extensively

1. them to have lost a lot of their RNA.
Another piece of evidence for thermoreduction comes from the
Prokaryotes genomes of prokaryotes:
can survive in
higher
• In eukaryotes, chromosomes are made of linear DNA.
temperatures
• In prokaryotes the genome is made of circular DNA.
because,
• Circular DNA is much less vulnerable to heat damage than
unlike
linear DNA, which starts to get 'split ends' at high
eukaryotes,
temperatures.
their DNA is in
a protective
circular • Circular chromosomes are conspicuously absent from
configuration. eukaryotes and their widespread incidence in prokaryotes
alone is best explained by the thermoreduction hypothesis.
For eukaryotes to maintain linear DNA genomes, they require a
special system for maintaining their ends:
• An enzyme called telomerase, which has both a protein and
an RNA component, does this job.
• Telomerase is common to all eukaryotes, suggesting it is
very ancient.
• It seems unlikely that eukaryotes with their linear genomes,
and many RNAs, including telomerase, could have emerged
from the 'sauna' of life.
• It is more likely that the organisms which first braved high
temperatures shed much of the evidence of their RNA world
ancestry along the way, as well as linear DNA genomes and
telomerase.
• Modern prokaryotes appear to have a 'hot history,' even
though many now live at moderate or even cold
temperatures.
Clues to the origin of life in your own body
Evolutionary biologists have traditionally studied the simplest
organisms they can find in order to learn more about the origins of
life. But simple doesn't necessarily mean ancient, so we should not
restrict our search purely to simple organisms. All organisms have
been evolving for 3.5 billion years or so, and the idea that there is
some obscure bug that time forgot which resembles ancient life on
Earth is outdated.
As Forterre's work shows, simplification has its merits, and it
seems that bacteria have lost a lot of the molecular fossils of our
ancient past. We know an enormous amount about the
Human cells,
biochemistry of our own cells, and although there's layer upon
which are
layer of complexity, hidden underneath it all are clues to the origins
eukaryotic,
of the earliest cells. How ironic it is that human cells harbor as
may harbor
many if not more secrets of the origins of life than the simple
secrets to the
bacteria! It's no wonder that evolutionary biologists are as excited
origin of life.
about the Human Genome Project as anyone else!
It is important to keep in mind that eukaryotic cells have continued
to evolve over time. While it is possible to uncover much about the

2. Were Bacteria the First Forms of Life on Earth?

3. learnmore links get involved glossary references back to top
Life in ocean extremes
Discovery network provides pictures of some of the creatures that live in and around
the extreme environment of deep sea vents.
http://dsc.discovery.com/convergence/blueplanet/photo/photo.html
The Origin of Life on Earth
Learn more about the RNA world and the current and historical ideas on the origin of
life on Earth by reading this thought provoking article by Leslie Orgel of the Salk
Institute for Biological Studies.
http://www.geocities.com/CapeCanaveral/Lab/2948/orgel.html
The Tree of Life page
A fun, interactive site that lets you surf up and down "The Tree of Life."
http://phylogeny.arizona.edu/tree/phylogeny.html
Human genome project information
Explore this comprehensive site, sponsored by the U.S. Department of Energy's
Office of Science, for information about the U.S. and worldwide Human Genome
Project. Second link takes you to their Primer on Molecular Genetics to learn more
about DNA and sequencing.
http://www.ornl.gov/hgmis/
http://www.ornl.gov/sci/techresources/Human_Genome/publicat/primer/toc.html
Eukaryotic Origins
Astrobiology Magazine article examines endosymbiosis (how bacteria is engulfed).
http://www.astrobio.net/news/article243.html
Astrobiology
The astrobiology website for Australia and New Zealand.
http://www.aao.gov.au/local/www/jab/astrobiology/
Common ancestor
News story on Biomednet (requires free log-in) describing a presentation by Anthony
Poole about the common ancestor being more like an eukaryote, presented at the
annual conference of the Society for Molecular Biology and Evolution, July 2001.
http://news.bmn.com/news/story?day=010711&story=2
Further reading (science journals)
» Doolittle W.F. 2000. "The nature of the universal ancestor and the evolution of the
proteome." Current Opinion in Structural Biology 10:355-358.
» Forterre P., Philippe H. 1999. "Where is the root of the universal tree of life?"
BioEssays 21:871-879.
» Penny D., Poole A. 1999. "The nature of the last universal common ancestor."
Current Opinion in Genetics and Development 9:672-677.
» Poole A., Jeffares D., Penny D. 1999. "Early evolution: prokaryotes, the new kids
on the block." BioEssays 21:880-889.
» Ridley M. 2000. "The search for LUCA." Natural History 11:82-85.
getinvolved links learn more glossary references back to top

4. The RNA Society
Biology students and scientists can join this scientific society to share research
results and information about emerging concepts in RNA. Membership application is
available on-line. There is a sliding membership fee.
http://www.rnasociety.org
For teachers: origin of life lessons
Interactive lessons focus on origin of life studies. These lessons are intended for use
in any high school biology course but many can be used in middle school, junior
college or lower division university levels.
http://www.indiana.edu/~ensiweb/orig.fs.html

5. authorglossary learn more get involved references back to top
Chromosome - A single length of DNA which contains many genes. Eukaryotes have
several different linear chromosomes in each cell, and each contains some of the
genes cells need for life (humans have 23). Prokaryotes usually have one circular
chromosome, which contains all their genes.
DNA - Deoxyribose nucleic acid (DNA) is built up of four types of units, adenine (A),
guanine (G), cytosine (C) and thymine (T), joined in a series. Genes are encoded by
the specific sequence of the DNA, and their products are usually proteins. For
example, the sequence ATG indicates the start point of a gene, and tells the cell that
the first amino acid in the protein is methionine.
DNA polymerases - A class of enzyme that synthesises DNA.
Enzyme - Enzymes speed up (catalyse) chemical reactions. Most enzymes are
proteins but a few are made from RNA.
Eukaryotes - Have cells with a nucleus that contains all the DNA. Eukaryotes
include plants, animals, fungi, amoeba, algae, and many other organisms.
Eukaryotes can be multicellular (many-celled) or single celled. Compare with
prokaryotes.
Genetic code - The language of genes. Three nucleotides together make a codon
which codes for an amino acid. For example GAG codes for the amino acid glutamine.
Ribosomes read the codons from an RNA copy of a gene, joining amino acids
together to make the protein encoded in that gene. The genetic code is the same for
all living things.
Genome - The collection of genes that make up an organism. These genes may be
on one
chromosome, or many.
Prokaryotes - Do not have a nucleus. There are two groups: bacteria and archaea,
which appear quite different from each other. All prokaryotes are single celled.
Protein - A chain of amino acids. The sequence of amino acids is specified by a
gene. Each protein is made by a ribosome, and it folds up into a specific shape that
is determined by the sequence of amino acids. The precise arrangement of amino
acids determines the properties of the protein. Some proteins are enzymes.
Ribosomes - Ribosomes are the cell's protein factories. Ribosomes read the genetic
code from the working RNA copies of genes, using these to synthesise the protein
encoded by the gene. Ribosomes have an RNA core, which is largely responsible for
protein production. The RNA core is stabilised by a protein scaffold.
RNA - Ribose nucleic acid (RNA) is similar to DNA. For a protein to be made from a
gene a working copy of the gene is made from RNA. The RNA is read by the
ribosome, which "translates" it into protein. Other RNA molecules are enzymes,
performing chemical reactions.
RNA world - A proposed stage early in the evolution of life in which RNA acted as
both genetic material and enzyme.

6. articlereferences learn more get involved glossary back to top
General References:
» Sidney Altman http://www.science.ca/scientists/scientistprofile.php?pID=3
» Tom Cech http://petunia.colorado.edu/projects.html
» Nobel Prize in Chemistry http://nobelprize.org/chemistry/laureates/1989/index.html
» Patrick Forterre is a leading scientist in France who has been studying eukaryotes and their role in the origin of
life http://www-archbac.u-psud.fr/LabHome/PForterre/ePF.html
» The University of Paris Sûd http://www.u-psud.fr/anglais.nsf/index.htm?OpenPage
» Human Genome Project http://www.ornl.gov/hgmis/