When we think about the evolution of the human body, we often picture a complex trajectory starting from relatively advanced organisms endowed with brains and sophisticated nervous systems. Yet a recent discovery challenges this view by suggesting that some of the fundamental mechanisms underlying our bodily organization may trace their roots to creatures much simpler and more distant from us: sea anemones.
These marine organisms, members of the phylum Cnidaria (which also includes jellyfish and corals), are far from being our closest relatives. They lack a brain, nor a centralized nervous system, and their bodies are organized radially, around a central point, in contrast to the bilateral symmetry that characterizes humans and the majority of complex animals. Yet, a study conducted by a team of researchers from the University of Vienna reveals that sea anemones use a molecular mechanism long associated with bilaterians to structure their bodies. This discovery could rewrite part of the history of animal evolution.
An ancient mechanism shared by distant lineages
The mechanism in question is the “Chordin-mediated BMP shuttle.” Behind this somewhat technical name lies a key process of embryonic development in bilaterians, that is, animals that exhibit left-right symmetry, such as humans, frogs, or insects. This system uses molecules called BMPs (Bone Morphogenetic Proteins) that act as messengers, telling cells their position in the embryo and the type of tissue they should become.
Concretely, the local inhibition of BMP by another molecule, Chordin, creates a concentration gradient within the developing organism. Depending on the amount of BMP present, the cells know whether they should form the central nervous system, the kidneys, or the ventral skin. This process thus establishes a dorsal-ventral axis that is fundamental for organizing the body structure of bilaterians.
Yet, researchers discovered that sea anemones, despite their very different organization, also use this same mechanism of a Chordin-mediated BMP shuttle. In other words, this process is not an innovation unique to bilaterians, but a much older evolutionary mechanism that would have existed long before the divergence between cnidarians and bilaterians.
An evolutionary origin dating back 600 million years
The divergence between cnidarians and bilaterians marks one of the major events in the evolutionary history of animals. These two groups have radically different body architectures and are separated by hundreds of millions of years of evolution, estimated at between 600 and 700 million years. The presence of the same molecular mechanism in both lineages suggests it was already present in their last common ancestor, a very ancient organism.
This hypothesis raises several exciting questions. First, it implies that the molecular foundations for organizing a complex body axis existed well before the appearance of bilaterians, which prompts a reevaluation of our understanding of the complexity of the earliest animals. Second, it challenges the idea that bilaterian structures formed entirely independently in each group, leaving open the possibility that the common ancestor of cnidarians and bilaterians itself possessed some form of primitive bilateral symmetry.
An ancient complexity well hidden
What this study highlights is that the apparent simplicity of sea anemones masks, in fact, an astonishingly sophisticated biological organization. Lacking a brain or a centralized nervous system, these animals nonetheless employ an advanced molecular system to organize their bodies from the embryonic stage. This ancestral complexity demonstrates that some evolutionary tools are so fundamental that they have been conserved, even shared, across very distant evolutionary branches.
David Mörsdorf, the study’s lead author, notes that this mechanism is not universal even among bilaterians. For instance, it is present in frogs but absent in fish, suggesting that it may have arisen and disappeared multiple times over the course of evolution. This plasticity and longevity make the Chordin-mediated BMP shuttle an excellent candidate for a key ancestral evolutionary mechanism in shaping the animal body.
Towards a new understanding of body evolution
This discovery is more than a mere scientific curiosity. It invites a rethink of the evolution of body development in animals, by integrating very ancient mechanisms shared among groups that seemed so distant until now. By studying organisms such as sea anemones, scientists can trace back to the deep origins of the biological processes that allowed the emergence of complex body forms, including our own.
Thus, perhaps deep in the oceans, within these brainless creatures, lies the true blueprint of the human body, written hundreds of millions of years ago.
