How a humble cockroach rewrote the history of New Caledonia

2024-06-22 08:27:5045568

Finding an insect species new to science can sometimes be very significant discovery. This is case for small cockroaches from the forests of New Caledonia, an island about 3,000 kilometres off Australia’s east coast.

Despite being different from Western home species, these cockroaches are still mere bugs that seem quite insignificant. But a few species studied during our exploratory field work in the late 1990s have revealed an outstanding story about this amazing island.

How a humble cockroach rewrote the history of New Caledonia

An ancient Noah’s ark?

The island is well known among scientists and nature lovers because it shelters amazing biodiversity. Tropical forests and mountains rise above beaches and the world’s largest lagoon. For naturalists, the archipelago made up of Grande Terre, the Loyalty Islands and many islets has still another virtue: its evolutionary history.

Map of New Caledonia and Vanuatu. Eric Gaba/Wikimedia, CC BY-SA

The largest island of the archipelago – Grande Terre – was long considered to be a small piece of land that separated from Australia about 80 million years ago, after the breakup of the ancient supercontinent of Gondwana approximately 180 million years ago.

Based on geological evidence, many scientists in the 1970s concluded – perhaps too quickly – that New Caledonian plants and animals may have been relics from those ancient times.

Some even listed the so-called living fossils that were supposed to have remained with little evolutionary change, such as many species of Araucaria(austral pines) and also Amborella trichopoda, a small tree supposedly left over from the first steps of diversification among flowering plants.

A few remarkable insects

We had some doubts about this beautiful theory, but decided to study the local fauna by focusing on a few groups of little-known insects. We were surprised to find a very rich and original biodiversity on an island that wasn’t particularly large (60,000 km²), including our small forest cockroaches from the group Angustonicus. But the best was still yet to come.

We built a tree of evolutionary relationships between all the species we captured (known as a phylogeny) that showed that the two species from the neighbouring Loyalty Islands – geologically very young at about 2 million years – diverged from all species of Grande Terre, which was supposedly ancient, at 80 million years and more. The result raised questions the supposed age of the island fauna.

Actually, because the two groups of organisms diverged from a common ancestor – called sister-groups by evolutionary biologists – they’re necessarily the same age, like fraternal twins born from the same mother. The species from Loyalty Islands, assumed to be as young as their island’s geologic understructure, forced us to consider the possibility that those of Grande Terre were, in fact, the same age and not some ancient relic.

What the geologists say

Puzzled by this result, we read the rich literature of Grande Terre’s geology, a place known for its many large nickel mines and well studied from this perspective.

Here lay our second surprise, consistent with the first. Under-layers of New Caledonia is indeed geologically very old (more than 80 million years) but the island later underwent many changes, being shaken up by the clashing of the Australian and Pacific tectonic plates.

Contractions and stretching at the limit of the two plates disturbed the old continental layers of New Caledonia, leading to several extended deep-water submersions of Grande Terre, the last one about 37 million years ago.

From a geological point of view, the island has an old substrate beneath a more recent surface layer. Direct evidence may be seen in the nickel mines: these metal-rich tropical soils were formed through the degradation of the oceanic crust (the bottom of ocean) brought up to the surface after the island’s last immersion.

Grande Terre can thus be considered an oceanic island because it emerged from the ocean, rather than a continental island, which separated from a continent, always remaining on the surface, such as Madagascar.

This suggests that the island’s fauna and flora actually developed quite recently, a hypothesis that went unnoticed by biologists who were unfamiliar with the geology literature.

The situation was thus more complex than what had previously been thought. New Caledonia was not a fragment of Gondwana that many had thought, drifting about since 80 million years ago with its ancient fauna and flora on board.

An old island with recent biodiversity

With this evidence in mind, we returned to study our cockroaches. We used them to question the myth of New Caledonia’s history in a 2005 publication that was considered controversial at the time.

For some colleagues, it was not pleasant to discuss the possibility that a small insect could challenge the seductive theory of a Gondwanan Noah’s Ark riding along straight from the time of the dinosaurs.

The southern coast of Grande Terre with Araucaria trees on the seaside. P. Grandcolas, CC BY

The idea that young fauna and flora could evolve on an old island has now gained support in the minds of scientists.

Since our first paper, studies have been carried out by dozens of teams. Our theory remains valid, as it has withstood a succession of tests. Our last published analysis references no less than 40 studies bearing on diverse groups of organisms – plants, insects, mollusks, lizards – that all support our point of view.

These studies date the beginning of the diversification of New Caledonian groups of the same age or later than the re-emersion of the island, about 37 million years.

To obtain such age estimates, modern evolutionary studies use mathematical methods to evaluate the probability that a succession of molecular divergences between relatives date from a specific time, under the control of related fossil species considered as calibration points.

Two strong conclusions came out of the last ten years of research. From a biological point of view, New Caledonia can no longer be considered as a kind of Noah’s ark. Instead, its different groups of organisms appear to be quite recent and result from a combination of dispersal over oceans and local evolution over 37 million years.

Retrospectively, it’s interesting to consider that such a deep change in minds concerning a famous hotspot of biodiversity was sparked by the study of a few cockroaches captured in a tropical forest. The discovery and analysis of an apparently insignificant species can thus sometimes have strong consequences, which is an idea that Charles Darwin would agree with, particularly after his trip to Galapagos islands.

This article was originally published in French





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