Vertebrate evolution kicked off in lagoons

Scientists have discovered that shallow, lagoon-like environments were the cradle for vertebrate evolution, giving rise to our distant ancestors.

A study led by palaeobiologist Lauren Sallan from the University of Pennsylvania, US, and published this week in the journal Science, solves one more piece in the long-standing puzzle of the evolutionary origin of vertebrates.

“Vertebrates encompass all animals with a backbone, from fish to humans,” explains palaeoecologist Catalina Pimiento in an accompanying article in the same journal. “How and when they evolved are questions that have been studied for centuries, revealing the origins and processes involved in anatomical innovations such as jaws, teeth, and paired appendages.”

The story kicks off 541 million years ago, when a dazzling burst of evolution called the Cambrian explosion gave rise to an incredible diversity of complex life, including the chordates, the group to which all vertebrates belong.{%recommended 2109%}

But the bridge from early chordates to modern vertebrates remains shaky. The very first vertebrates – fish – are thought to have evolved in the mid-Palaeozoic era around 480 million years ago, but the fossil record from this time contains only elusive fragments of these ancestral species. By 420 million years ago, the record shows a huge proliferation of fish – so what happened in the intervening 60 million? 

Over the past century, researchers in several fields have worked together to build a more complete picture of the transition.

Some clues have come from studying the genomes of species positioned at crucial points along the evolutionary tree, such as jawless fish, including lampreys and hagfish, and the marine invertebrates known as lancelets or Amphioxi. These are the closest living relatives of the ancestors of vertebrates. The differences between them hold hints about vertebrate evolution.

Other clues have come from neurobiology, anatomy, developmental biology, palaeontology and DNA-sequence data. Piece by piece, these have improved our understanding of how and when our ancestors arose.

But exactly where vertebrates evolved is a less-explored question and has long been debated. 

Fossils from the mid-Palaeozoic are mostly fragmented and limited in distribution, and different groups of them suggest different ancestral habitats, from freshwater to open ocean. Scientists have also wondered whether coral reefs – which are so important for biodiversity today – might have been the cradle for vertebrate evolution, but decades of fossil searches have come up empty. 

The study by Sallan and colleagues analyses the entire early environmental record of primitive fish – comprising 2728 previously published records – to create a huge new dataset. This allowed them to reconstruct these ancestral habitats, using mathematical models to fill in the gaps.

“Our work shows that almost every major vertebrate division, from the earliest armoured jawless fish all the way up through sharks and our own ancestors, all started out right near the beach, far inshore of the reef,” Sallan says. 

These fragile near-shore environments – either intertidal areas or permanently shallow lagoons – explain why so few early fish fossils are found intact: waves likely blasted them into tiny fragments.

Importantly, the study also shows that these restricted areas remained cradles of diversification for a long time.

“Even as older groups spread out, newer groups were also appearing at the shoreline,” Sallan notes.

The species that emerged diversified and gained a variety of adaptations that allowed them to compete for new habitats.

“Those that were robust and heavily armoured tended to stay in shallow waters or migrate into fresh waters, whilst those that were more weakly armoured were more easily able to move into open oceans,” says co-author Ivan Sansom from the University of Birmingham in the UK.

Some even moved onto dry land, becoming the first land-dwelling vertebrates.

The research provides much-needed environmental context to the origin of vertebrates. As Sallan explains, the ecology of animals has a huge influence on their form and evolutionary history. 

“Habitat in particular affects everything from available resources to population sizes to the identity of predators and competitors,” she says, “and so has a huge effect on the potential diversity and evolutionary pathways of groups through time.”