2004/10/28 by A. Richard Palmer · 414 citations
Mathematics · Neuroscience · #Asymmetry #Biological evolution #Biology #Body plan #Chordate #Evolutionary biology #Evolutionary developmental biology #Gene #Genetics #Hemispheric Asymmetry in Neuroscience #Homology (biology) #Morphological variations and asymmetry #Novelty #Phenotype #Phylogenetic tree #Phylogenetics #Vertebrate
paper · doi:10.1126/science.1103707
published in Science 306(5697), 828-833 (American Association for the Advancement of Science)
openalex publication_date 2004/10/28 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/23
Because of its simplicity, the binary-switch nature of left-right asymmetry permits meaningful comparisons among many different organisms. Phylogenetic analyses of asymmetry variation, inheritance, and molecular mechanisms reveal unexpected insights into how development evolves. First, directional asymmetry, an evolutionary novelty, arose from nonheritable origins almost as often as from mutations, implying that genetic assimilation ("phenotype precedes genotype") is a common mode of evolution. Second, the molecular pathway directing hearts leftward-the nodal cascade-varies considerably among vertebrates (homology of form does not require homology of development) and was possibly co-opted from a preexisting asymmetrical chordate organ system. Finally, declining frequencies of spontaneous asymmetry reversal throughout vertebrate evolution suggest that heart development has become more canalized.