“Man is but a worm” was the title of a famous caricature
ideas in Victorian England. Now, 120 years later, a molecular analysis of
mysterious marine creatures unexpectedly reveals our cousins as worms, indeed.
An international team of researchers, including a
neuroscientist from the Univ.
of Florida, has produced more
evidence that people have a close evolutionary connection with tiny,
flatworm-like organisms scientifically known as “Acoelomorphs.”
The research in Nature
offers insights into brain development and human diseases, possibly shedding
light on animal models used to study development of nerve cells and complex
neurodegenerative diseases such as Alzheimer’s and Parkinson’s.
“It was like looking under a rock and finding something
unexpected,” said Leonid L. Moroz, a professor in the department of
neuroscience with the UF College of Medicine. “We’ve known there were very
unusual twists in the evolution of the complex brains, but this suggests the
independent evolution of complex brains in our lineage versus invertebrates,
for example, in lineages leading to the octopus or the honeybee.”
The latest research indicates that of the five animal
phyla, the highest classification in our evolutionary neighborhood, four
contain worms. But none are anatomically simpler than “acoels,” which have no
brains or centralized nervous systems. Less than a few millimeters in size,
acoels are little more than tiny bags of cells that breathe through their skin
and digest food by surrounding it.
Comparing extensive genome-wide data, mitochondrial
genes and tiny signaling nucleic acids called microRNAs, the researchers
hailing from six countries determined a strong possibility that acoels and their
kin are “sisters” to another peculiar type of marine worm from northern seas,
From there, like playing “Six Degrees of Kevin Bacon,”
the branches continue to humans.
“If you looked at one of these creatures you would say,
‘what is all of this excitement about a worm?’” said Richard G. Northcutt, a
professor of neurosciences at Scripps Institution of Oceanography, who was not
involved in the study. “These are tiny animals that have almost no anatomy,
which presents very little for scientists to compare them with. But through
genetics, if the analysis is correct—and time will tell if it is—the study has
taken a very bothersome group that scientists are not sure what to do with and
says it is related to vertebrates, ourselves and echinoderms (such as
“The significance of the research is it gives us a
better understanding of how animals are related and, by inference, a better
understanding of the history of the animals leading to humans,” Northcutt said.
Scientists used high-throughput computational tools to
reconstruct deep evolutionary relationships, apparently confirming suspicions
that three lineages of marine worms and vertebrates are part of a common
evolutionary line called “deuterostomes,” which share a common ancestor.
“The early evolution of lineages leading to vertebrates,
sea stars and acorn worms is much more complex than most people expect because
it involves not just gene gain, but enormous gene loss,” said Moroz, who is
affiliated with the Whitney Laboratory for Marine Bioscience and UF’s McKnight
Brain Institute. “An alternative, yet unlikely, scenario would be that our
common ancestor had a central nervous system, and then just lost it, still
remaining a free living organism.
Understanding the complex cellular rearrangements and
the origin of animal innovations, such as the brain, is critically important
for understanding human development and disease, Moroz said.
“We need to be able to interpret molecular events in the
medical field,” he said. “Is what’s happening in different lineages of neuronal
and stem cells, for example, completely new, or is it reflecting something that
is in the arrays of ancestral toolkits preserved over more than 550 million
years of our evolutionary history? Working with models of human disease, you
really need to be sure.”