The story of a groundbreaking study that used molecular data to redraw evolutionary relationships and sparked decades of scientific refinement.
Published: February 12, 1988 | Authors: Katherine Field, Rudy Raff, et al.
Have you ever looked at a starfish and wondered how it is related to you? For centuries, biologists pieced together the evolutionary relationships of animals by carefully comparing their anatomy and embryonic development. But on February 12, 1988—Charles Darwin's birthday—a quiet revolution began. A team of scientists led by Katherine Field and Rudy Raff published a paper in the journal Science that did something unprecedented: it used the molecular alphabet of genes to redraw the entire animal family tree 1 6 .
This pioneering work, often referred to as "Field et al.," ignited a new field of science and set off a decades-long quest for the true tree of life, a journey of discovery and debate that scientists now call "Field et al. Redux" 1 .
The shift from anatomical comparisons to genetic sequencing marked a turning point in evolutionary biology.
Published on Darwin's birthday, the paper honored the past while pointing toward the future of evolutionary studies.
Traditionally, scientists reconstructed evolutionary history, or phylogeny, by comparing physical characteristics. Animals with similar structures were presumed to be close relatives. This approach, however, had limitations. What if unrelated species evolved similar features independently to adapt to the same environment? This is known as convergent evolution.
The Field et al. paper introduced a powerful new tool to solve this puzzle: molecular phylogenetics 1 . The core idea is simple—the more similar the DNA sequences of two species are, the more recently they shared a common ancestor.
Found in all living cells, from bacteria to humans
Provides a stable record of deep evolutionary splits
Helps distinguish between recently diverged groups
The researchers chose to sequence a particular gene, the Small Subunit Ribosomal RNA (SSU), for several clever reasons 1 :
This molecular approach promised an objective way to test long-held assumptions about animal relationships and reveal the hidden branches of our shared evolutionary history.
The 1988 Field et al. study was a monumental technical achievement. At a time when sequencing DNA was a slow and laborious process, the team used a novel method to directly read the sequence of the SSU rRNA gene from a diverse set of animals, including jellyfish, flatworms, earthworms, sea urchins, and humans 1 6 .
The experimental procedure was groundbreaking for its time:
The researchers gathered biological samples from a wide array of animal species, ensuring a representative look across the metazoan (animal) kingdom 1 .
They used a technique pioneered by Norman Pace's group that involved reverse transcriptase to quickly determine the sequence of the SSU rRNA gene 1 . This was the high-throughput technology of its day.
The different gene sequences were then carefully lined up against each other to identify which positions in the genetic code were the same and which were different 1 .
Finally, they used early computer programs to analyze the aligned sequences. They primarily used distance-based methods, which group species based on the overall similarity of their sequences, to infer the most likely evolutionary tree 1 .
Used reverse transcriptase for direct RNA sequencing, bypassing the need for cloning.
Employed early phylogenetic software to build trees from molecular data.
The results were both stunning and controversial. The Field et al. tree confirmed some classical views but also delivered shocking upsets 1 :
The study successfully grouped obviously related organisms, such as vertebrates within the chordates.
The analysis also produced two significant mistakes. It suggested that cnidarians (like jellyfish) and bilaterians (animals with bilateral symmetry) had independent origins, making animals polyphyletic. It also placed flatworms (Platyhelminthes) in a basal position, far from their correct location among the lophotrochozoans (a group including mollusks and annelids) 1 .
We now understand these errors were largely due to systematic bias. The early models of evolution used in their analysis were too simple and couldn't account for the fact that some species' genes evolve much faster than others. This "long-branch attraction" caused fast-evolving species (like the flatworm Dugesia) to be mistakenly placed at the base of the tree 1 .
| Finding Category | Specific Example in the Paper | Modern Interpretation |
|---|---|---|
| Correct Grouping | Vertebrates grouped together within Chordates. | Still accepted today; validated by later studies. |
| Major Error | Cnidarians and bilaterians shown as having independent origins (polyphyletic Metazoa). | Incorrect; all animals are monophyletic, sharing a single common ancestor. |
| Major Error | Flatworm (Dugesia) placed basal to all other bilaterians. | Incorrect; flatworms are part of the Lophotrochozoa, alongside mollusks and annelids. |
| Within-Group Error | Bivalve mollusks (clams) not shown as closely related to other mollusks. | Incorrect; mollusks are a monophyletic group. |
| Method to Reduce Error | How It Works | Application in "Field et al. Redux" |
|---|---|---|
| More Data | Using longer gene sequences or data from hundreds/thousands of genes. | Re-analysis used the full-length SSU gene instead of partial sequences 1 . |
| Better Taxon Sampling | Carefully selecting species to avoid "long-branched" fast evolvers. | Replacing the fast-evolving flatworm Dugesia with a slower-evolving relative corrected its position 1 . |
| Superior Evolutionary Models | Using complex statistical models that account for different rates of evolution. | Maximum Likelihood methods available in 1988, but too computationally expensive, produced a more accurate tree 1 . |
Interactive Phylogenetic Tree Visualization
This interactive diagram would show the original Field et al. tree alongside the modern corrected version, highlighting the major changes in understanding.
The story of Field et al. Redux is a powerful testament to the self-correcting nature of science. The original paper was not an endpoint but a catalyst. It proved that molecular data held the key to unraveling deep evolutionary history and inspired a generation of scientists to refine its methods 1 .
Over the following 25 years, the three pillars of modern phylogenetics—ever-larger datasets, careful species selection, and increasingly sophisticated models—solidified our understanding into the "new animal phylogeny" we know today 1 .
This journey from a single groundbreaking gene sequence to entire genome comparisons shows how scientific knowledge evolves. It reminds us that every revolutionary finding is a starting point, inviting future explorers to question, test, and refine our picture of the natural world.
The quest that Field et al. began continues today, as scientists use their powerful tools to fill in the remaining blanks on the map of life, tracing the connections between all living things with ever-greater clarity.