Acorn worms are the unlikely missing link between humans and starfish
In short: Hemichordates, or acorn worms, are marine invertebrates that hold the key to understanding our own evolutionary origins. By sharing a common ancestor with both echinoderms and chordates, these soft-bodied creatures reveal how complex body plans and filter-feeding lifestyles first emerged in the earliest deuterostomes hundreds of millions of years ago.
Hemichordates, specifically the enteropneust acorn worms, are essential for mapping the evolution of deuterostomes—a group that includes humans and starfish. These animals possess a three-part body plan consisting of a proboscis, collar, and trunk. While 19th-century zoologists initially grouped them with chordates due to their pharyngeal gill slits and midline nerve cords, modern molecular phylogenies confirm they are actually sister phyla with echinoderms. Together, these two groups form the ambulacrarians, which share a deep evolutionary history with chordates.
The genomic analysis of two acorn worm species, Saccoglossus kowalevskii and Ptychodera flava, has provided a window into the ancestral deuterostome genome. Despite diverging over 370 million years ago, these species exhibit similar gene content and conserved genomic clusters. Notably, they possess a specific cluster of four transcription factor genes linked to the development of pharyngeal gill slits. These structures were a major innovation for early filter-feeding ancestors, though they were later lost in modern echinoderms and amniotes. The expression of the pax1/9 gene in these slits provides strong molecular evidence for their shared ancestry with chordates.
Beyond their anatomy, hemichordates offer a unique perspective on developmental diversity. S. kowalevskii develops directly into a juvenile, while P. flava undergoes a complex metamorphosis after spending months as a pelagic larva. Studying these varied developmental trajectories, alongside their conserved non-coding genomic elements, allows researchers to infer the features of the last common deuterostome ancestor. This ancestor likely utilized pharyngeal gill slits for efficient suspension feeding, a lifestyle that set the stage for the subsequent radiation of hemichordates, echinoderms, and chordates across the Cambrian and Precambrian periods.
The study of these genomes integrates morphological, physiological, and embryological data to resolve long-standing debates in animal phylogeny. By comparing these marine invertebrates with diverse bilaterian genomes, scientists can identify the specific gene novelties that allowed deuterostomes to thrive. This research highlights how ancient genomic innovations continue to inform our understanding of the complex body plans found in the animal kingdom today.
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Sources
- Summary by Intellect · source: Hemichordate genomes and deuterostome origins - Nature
- Also: Hemichordate
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