The international study, co-led by two joint research centers of the CSIC and Harvard University, explains how the “up and down” orientation of vertebrate limbs was established more than 400 million years ago
The study identifies a key switch in this process that has been functioning without significant changes for at least 450 million years
A study co-led by the Spanish National Research Council (CSIC), an agency under the Ministry of Science, Innovation, and Universities, has taken a key step toward understanding how vertebrate limbs evolved. The study, published in Current Biology, identifies the ancestral genetic mechanism that determines which side is the dorsal (the “top”) side of fish fins, and it shows that this system already existed long before vertebrates adapted to life on land, more than 400 million years ago.
“When we look at our hand, the distinction between the back (the part with the fingernails) and the palm (with its creases and pads) seems completely natural to us. And it’s not just an aesthetic issue: that difference is essential for our limbs to function properly and enables everyday actions such as bending our fingers, grasping objects, or placing our foot on the ground,” notes Marian Ros, a CSIC researcher at the Institute of Biomedicine and Biotechnology of Cantabria (IBBTEC-CSIC-UNICAN) and one of the study’s lead authors. However, scientists did not know when or how this asymmetry arose over the course of evolution, nor whether it already existed in fish fins—from which vertebrate limbs are derived.
The gene that determines “up”
The study shows that the Lmx1b gene, already known to be the dorsal determinant in mice, chickens, and humans, performs exactly the same function in the pectoral fins of zebrafish (Danio rerio). “When this gene was knocked out using the CRISPR-Cas9 genome editing technique, the upper part of those fins developed the same structures that normally appear on the underside.”
“We scientists call this result a ‘double ventral’ appendage—a kind of fin with two ventral sides and no dorsal side, which would be like having a hand with two palms,” explains the researcher.
But it’s not just about the gene. Genes don’t act alone: they need instructions telling them when and where to activate. “The team found that the set of DNA regulatory sequences (LARM), which act as a Lmx1b-specific switch in the dorsal region of the limbs, are present and functionally equivalent from sharks to bony fish and in all terrestrial vertebrates, including mice and humans. In other words, this switch has been functioning without any significant changes for at least 450 million years, dating back to the common ancestor of all vertebrates with jaws.
An innovation unique to the paired fins
The study went even further. It is known that the fins from which our limbs are derived (the paired fins, such as the pectoral and pelvic fins) repurposed genetic mechanisms that were originally developed to form the medial fins, such as the dorsal fin of a fish. However, regulation by LARMs is unique to the paired fins and does not control gene expression in the medial fins. This means that the LARM “switch” is not a reused ancestral trait, but rather an evolutionary innovation that arose specifically with the origin of paired limbs in jawed vertebrates, which would later evolve into terrestrial limbs.
Stream minnows: a natural experiment in evolution
The study also includes a particularly striking discovery. Certain freshwater fish known as stream loaches or butterfly loaches, which are adapted to living clinging to rocks in fast-flowing rivers, have naturally developed pectoral fins that help them grip surfaces, with morphologies that mimic the “double ventral” fins created in the laboratory.
Upon analyzing the DNA of these fish, the team found that the LARM sequences in their genome, specifically, have undergone specific and accelerated changes in that very region. In other words, natural evolution has modified the very genetic switch that the scientists were studying, and that modification is directly reflected in the unique morphology of their fins. This is the first known example of this type of adaptive variation in the limbs of vertebrates along the dorso-ventral axis…
Nail-Patella Syndrome
In addition to explaining a key aspect of vertebrate anatomy, this study helps us understand diseases such as nail-patella syndrome, in which patients have mutations in the LMX1B gene or the LARM switches, leading to the loss of dorsal limb features such as nails and patellas.
International Collaboration
The study is the result of a large-scale international scientific collaboration. The research is co-led by Dr. Marian A. Ros (Institute of Biomedicine and Biotechnology of Cantabria, part of the CSIC and the University of Cantabria), Dr. Juan J. Tena (Andalusian Center for Developmental Biology, part of the CSIC and Pablo de Olavide University in Seville), and Dr. Matthew P. Harris (Boston Children’s Hospital / Harvard University).
The first two authors are Dr. M. Brent Hawkins (Harvard University) and Dr. Sofía Zdral (University of Cantabria). The research team also includes members from the University of Houston, Boston College, Comenius University in Bratislava (Slovakia), the University of Colorado, and the University of Salamanca. The research was funded by the U.S. National Institutes of Health (NIH) and the National Science Foundation (NSF), the Slovak Scientific Agency, Spain’s Ministry of Science and Innovation, the University of Cantabria, among others. It also received support from the “LifeHUB Jump-Starta Scientific Exchanges” program to facilitate the visit and collaboration among the participating CSIC groups.
Reference:
Hawkins, M. B., Zdral, S., Naranjo, S., Juliá, M., Sánchez-Martín, M., Daane, J. M., … & Ros, M. A. (2026). The origin and evolution of dorsal determination mechanisms in vertebrate paired appendages. Current Biology. DOI: https://doi.org/10.1016/j.cub.2026.05.046