Why We All Start as Fish

Have you ever looked at a human embryo and thought, “That kind of looks like a fish”? If you haven’t, you might be surprised to learn that, in a way, we all begin our lives like this. The idea that our embryonic development reflects our evolutionary past is fascinating, revealing deep connections among all living creatures. While we don’t evolve from fish into humans in the womb, our early development holds clues about the ancient origins of our species (Gilbert, 2013). Exploring our biology and discovering why we all start life resembling our aquatic ancestors is crucial; after all, mental health often stems from our evolutionary traits.

The Evolutionary Blueprint in Every Human Embryo

Biologist Ernst Haeckel popularized the theory that "ontogeny recapitulates phylogeny"—the notion that embryonic development mirrors a species' evolutionary history—in the 19th century. Although this idea, in its strictest form, has been largely discredited, aspects of it remain valid (Hall, 1999). In their earliest stages, human embryos exhibit striking similarities to fish embryos, demonstrating how deeply our evolutionary history is embedded in our biology.

• The gills that aren’t gills: One of the most fascinating clues connecting us to our fishy past is the presence of pharyngeal arches, commonly known as "gill slits." Tiny ridges form in the throat during the fourth or fifth week of human embryonic development. If we were fish, these structures would develop into actual gills. However, in humans, these arches transform into parts of the jaw, ears, and throat (Raff, 1996). The fact that our bodies initially form structures that could have been gills—before reshaping them into something more suitable for a land-dwelling mammal—serves as an astonishing reminder of our evolutionary roots.
• The tail we all had: During our early weeks in the womb, human embryos develop something entirely unexpected—a tail! For a time, this tail is prominent and fully visible. However, as development continues, it shrinks and is absorbed into the body, eventually forming the tailbone (coccyx) that we all carry today (Zimmer, 2001). While other primates retain functional tails, humans evolved away from them as we adopted upright walking. This temporary tail during our development is a ghostly echo of a time when our ancestors needed it for balance and movement.
• Webbed fingers and toes: Another surprising phase in human development is the emergence of webbed hands and feet. In fish, these webbed structures transform into fins, but human embryos ultimately develop distinct fingers and toes. This process is controlled by programmed cell death (apoptosis), which eliminates the webbing and forms our hands and feet into the shape we recognize today (Carroll, 2005). In rare genetic cases, babies are born with webbed fingers or toes (a condition known as syndactyly), which further highlights our evolutionary links to aquatic creatures.

Why Does This Matter?

It might be amusing to think of ourselves as tiny fish-like creatures before fully developing, but this knowledge is more than just a biological curiosity. Understanding how our development echoes our evolutionary history is essential for several reasons:

• It helps us understand birth defects and genetic disorders: Scientists can better understand why certain birth defects occur by studying how human embryos develop and comparing them to our evolutionary past. For example, if something disrupts the regular transformation of pharyngeal arches, it can lead to conditions like cleft palate or ear malformations. Understanding our evolutionary past helps medical researchers pinpoint why these issues arise and how they might be prevented or treated (Gilbert, 2013).
• It highlights our connection to other life forms: Recognizing that human development follows patterns seen in other animals reminds us that we are not separate from nature—we are deeply connected to it. Evolution is not just an abstract scientific concept; it is actively playing out in the earliest moments of our existence. This perspective fosters a greater appreciation for the diversity of life and our shared ancestry with all living beings (Hall, 1999).
• It shows how evolution shapes our bodies even today: Even though we have evolved into modern humans, evolutionary remnants still linger in our biology. From our appendix (once beneficial for digesting rigid plant material) to goosebumps (a leftover reflex from when we had thicker body hair for insulation), understanding these evolutionary leftovers helps us appreciate why our bodies are the way they are (Zimmer, 2001).

The Story of Us—Written in Our DNA

Our journey from a single fertilized egg to a fully developed baby reflects millions of years of evolutionary history. While we might not be fish, our early development reminds us of our origins. But this evolutionary imprint isn’t biological—it shapes our psychology, behavior, and everyday lives.

Consider our instincts, emotions, and decision-making. Once crucial for escaping predators, the fight-or-flight response still influences our reactions to stress, even if the threats today are work deadlines or social conflicts. Our social bonds, shaped by the need for cooperation in early human tribes, still define our relationships, friendships, and sense of belonging.

Even our capacity for change and growth mirrors evolution itself. Just as life adapted to new environments over millennia, we constantly adapt to new challenges, learning and evolving within our own lifetimes. The resilience, curiosity, and drive to survive embedded in our DNA push us forward, whether in personal struggles or collective progress.

So, the next time you reflect on human evolution, remember—our story isn’t just in fossils or textbooks. It’s alive in our thoughts, emotions, and behaviors. It’s written in every challenge we overcome, every connection we form, and every adaptation we make. Evolution isn’t just behind us—it’s within us, shaping who we are and who we will become.