Scientists call it LUCA, the Last Universal Common Ancestor,
but they don’t know much about this great grandparent of all living things.
Many believe LUCA was little more than a crude assemblage of molecular parts, a
chemical soup out of which evolution gradually constructed more complex forms.
Some scientists still debate whether it was even a cell.
New evidence suggests that LUCA was a sophisticated organism
after all, with a complex structure recognizable as a cell, researchers report.
Their study appears in Biology Direct.
The study builds on several years of research into a
once-overlooked feature of microbial cells, a region with a high concentration
of polyphosphate, a type of energy currency in cells. Researchers report that
this polyphosphate storage site actually represents the first known universal
organelle, a structure once thought to be absent from bacteria and their
distantly related microbial cousins, the archaea. This organelle, the evidence
indicates, is present in the three domains of life: bacteria, archaea and
eukaryotes (plants, animals, fungi, algae, and everything else).
The existence of an organelle in bacteria goes against the
traditional definition of these organisms, says University of Illinois
crop sciences professor Manfredo Seufferheld, who led the study.
“It was a dogma of microbiology that organelles weren’t
present in bacteria,” he says. But in 2003 in a paper in the Journal of Biological Chemistry,
Seufferheld and colleagues showed that the polyphosphate storage structure in
bacteria (they analyzed an agrobacterium) was physically, chemically, and
functionally the same as an organelle called an acidocalcisome found in many
single-celled eukaryotes.
Their findings, the authors wrote, “suggest that
acidocalcisomes arose before the prokaryotic (bacterial) and eukaryotic
lineages diverged.” The new study suggests that the origins of the organelle
are even more ancient.
The study tracks the evolutionary history of a protein
enzyme (called a vacuolar proton pyrophosphatase, or V-H+PPase) that is common
in the acidocalcisomes of eukaryotic and bacterial cells.
By comparing the sequences of the V-H+PPase genes from
hundreds of organisms representing the three domains of life, the team
constructed a “family tree” that showed how different versions of the enzyme in
different organisms were related. That tree was similar in broad detail to the
universal tree of life created from an analysis of hundreds of genes. This
indicates, the researchers say, that the V-H+PPase enzyme and the
acidocalcisome it serves are very ancient, dating back to the LUCA, before the
three main branches of the tree of life appeared.
“There are many possible scenarios that could explain this,
but the best, the most parsimonious, the most likely would be that you had
already the enzyme even before diversification started on Earth,” says study coauthor
Gustavo Caetano-Anollés, a professor of crop sciences and an affiliate of the Institute
for Genomic Biology at Illinois. “The protein was there to begin with and was
then inherited into all emerging lineages.”
“This is the only organelle to our knowledge now that is
common to eukaryotes, that is common to bacteria and that is most likely common
to archaea,” Seufferheld says. “It is the only one that is universal.”
The study lends support to a hypothesis that LUCA may have
been more complex even than the simplest organisms alive today, says James
Whitfield, a professor of entomology at Illinois and a coauthor on the study.
“You can’t assume that the whole story of life is just
building and assembling things,” Whitfield says. “Some have argued that the
reason that bacteria are so simple is because they have to live in extreme
environments and they have to reproduce extremely quickly. So they may actually
be reduced versions of what was there originally. According to this view,
they’ve become streamlined genetically and structurally from what they
originally were like. We may have underestimated how complex this common
ancestor actually was.”
