According to a new study, fossils of a tiny sea creature with a delicately preserved nervous system solve a century-old debate over how the brain evolved in arthropods, the most species-rich group in the animal kingdom. Combining detailed anatomical studies of the fossilized nervous system with analyses of gene expression patterns in living descendants, they conclude that a shared blueprint of brain organization has been maintained from the Cambrian until today.
From: University of Arizona
November 25, 2022 -- Fossils
of a tiny sea creature that died more than half a billion years ago may compel
a science textbook rewrite of how brains evolved.
A study published in Science
-- led by Nicholas Strausfeld,a Regents Professor in the University of Arizona
Department of Neuroscience, and Frank Hirth, a reader of evolutionary
neuroscience at King's College London -- provides the first detailed
description of Cardiodictyon catenulum, a wormlike animal preserved
in rocks in China's southern Yunnan province. Measuring barely half an inch
(less than 1.5 centimeters) long and initially discovered in 1984, the fossil
had hidden a crucial secret until now: a delicately preserved nervous system,
including a brain.
"To our knowledge,
this is the oldest fossilized brain we know of, so far," Strausfeld said.
Cardiodictyon belonged to an extinct group of animals known as
armored lobopodians, which were abundant early during a period known as the
Cambrian, when virtually all major animal lineages appeared over an extremely
short time between 540 million and 500 million years ago. Lobopodians likely
moved about on the sea floor using multiple pairs of soft, stubby legs that
lacked the joints of their descendants, the euarthropods -- Greek for
"real jointed foot." Today's closest living relatives of lobopodians
are velvet worms that live mainly in Australia, New Zealand and South America.
A debate going back to
the 1800s
Fossils of Cardiodictyon reveal
an animal with a segmented trunk in which there are repeating arrangements of
neural structures known as ganglia. This contrasts starkly with its head and
brain, both of which lack any evidence of segmentation.
"This anatomy was
completely unexpected because the heads and brains of modern arthropods, and
some of their fossilized ancestors, have for over a hundred years been
considered as segmented," Strausfeld said.
According to the
authors, the finding resolves a long and heated debate about the origin and
composition of the head in arthropods, the world's most species-rich group in
the animal kingdom. Arthropods include insects, crustaceans, spiders and other
arachnids, plus some other lineages such as millipedes and centipedes.
"From the 1880s,
biologists noted the clearly segmented appearance of the trunk typical for
arthropods, and basically extrapolated that to the head," Hirth said.
"That is how the field arrived at supposing the head is an anterior
extension of a segmented trunk."
"But Cardiodictyon shows
that the early head wasn't segmented, nor was its brain, which suggests the
brain and the trunk nervous system likely evolved separately," Strausfeld
said.
Brains do fossilize
Cardiodictyon was part of the Chengjiang fauna, a famous
deposit of fossils in the Yunnan Province discovered by paleontologist
Xianguang Hou. The soft, delicate bodies of lobopodians have preserved well in
the fossil record, but other than Cardiodictyon none have been
scrutinized for their head and brain, possibly because lobopodians are
generally small. The most prominent parts of Cardiodictyon were
a series of triangular, saddle-shaped structures that defined each segment and
served as attachment points for pairs of legs. Those had been found in even
older rocks dating back to the advent of the Cambrian.
"That tells us
that armored lobopodians might have been the earliest arthropods,"
Strausfeld said, predating even trilobites, an iconic and diverse group of
marine arthropods that went extinct around 250 million years ago.
"Until very
recently, the common understanding was 'brains don't fossilize,'" Hirth
said. "So you would not expect to find a fossil with a preserved brain in
the first place. And, second, this animal is so small you would not even dare
to look at it in hopes of finding a brain."
However, work over the
last 10 years, much of it done by Strausfeld, has identified several cases of
preserved brains in a variety of fossilized arthropods.
A common genetic ground
plan for making a brain
In their new study, the
authors not only identified the brain of Cardiodictyon but
also compared it with those of known fossils and of living arthropods,
including spiders and centipedes. Combining detailed anatomical studies of the
lobopodian fossils with analyses of gene expression patterns in their living
descendants, they conclude that a shared blueprint of brain organization has
been maintained from the Cambrian until today.
"By comparing
known gene expression patterns in living species," Hirth said, "we
identified a common signature of all brains and how they are formed."
In Cardiodictyon,
three brain domains are each associated with a characteristic pair of head
appendages and with one of the three parts of the anterior digestive system.
"We realized that
each brain domain and its corresponding features are specified by the same
combination genes, irrespective of the species we looked at," added Hirth.
"This suggested a common genetic ground plan for making a brain."
Lessons for vertebrate
brain evolution
Hirth and Strausfeld
say the principles described in their study probably apply to other creatures
outside of arthropods and their immediate relatives. This has important
implications when comparing the nervous system of arthropods with those of
vertebrates, which show a similar distinct architecture in which the forebrain
and midbrain are genetically and developmentally distinct from the spinal cord,
they said.
Strausfeld said their
findings also offer a message of continuity at a time when the planet is
changing dramatically under the influence of climatic shifts.
"At a time when
major geological and climatic events were reshaping the planet, simple marine
animals such as Cardiodictyon gave rise to the world's most
diverse group of organisms -- the euarthropods -- that eventually spread to
every emergent habitat on Earth, but which are now being threatened by our own
ephemeral species."
Funding for this work
was provided by the National Science Foundation, the University of Arizona
Regents Fund, and the UK Biotechnology and Biological Sciences Research
Council.
https://www.sciencedaily.com/releases/2022/11/221125132137.htm
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