Insects Pester Darwinian Story

It’s enough to bug any Darwinian: where did the insects come from?  Here are some problems right off the bat sonar:

• Insects are fantastically diverse.
• Insects are among the most successful animals.
• There are no insect fossils earlier than the Devonian (evolutionary date: 410 million years ago).
• The earliest segmented body plans appeared in the Cambrian (511 million years ago).
• There are no marine insects, but the first segmented Cambrian animals were marine organisms.

Now, visualize the following: (1) an insect with six legs, (2) a spider with eight legs, (3) a centipede with 15 to 173 pairs of legs, and (4) a crab with 10 legs, two of which are claws.  Your job is to organize these into an evolutionary story of common descent.  It’s enough to challenge the most committed Darwinist, as the opening to a paper in Science demonstrates:¹

Although hexapods–those arthropods having six legs, including insects–are the most diverse group of contemporary animals in terms of biological niches and number of species, their origin is highly debated.  A key problem is the almost complete absence of fossils that connect hexapods to the other major arthropod subphyla, namely Crustacea, Myriapoda (such as centipedes and millipedes), and Chelicerata (such as scorpions and spiders).  Over the years, hexapods (insects, springtails, proturnas, and diplurans) have been phylogenetically linked to all of these major arthropod taxa.

By this, the authors mean evolutionists have attempted to link the insects to all these groups (see also 05/16/2002).  Glenner et al described the latest theory: that insects descended from crustaceans (crabs, lobsters, shrimp, barnacles, etc.), particularly from a group of freshwater branchiopods including fairy shrimp and water fleas.  They based this on molecular studies, Hox gene behavior and the emergence of these ancestors a few million years before the rise of insects in freshwater habitats.  Another piece of circumstantial evidence comes from a real estate boom supposedly taking place throughout the animal kingdom 423 to 416 million years ago:

The successful colonization of the terrestrial environment by hexapods seems to coincide with other major groups of land pioneering animals such as the chelicerates and the myriapods in the Late Silurian and the tetrapods (amphibians, reptiles, birds, and mammals) in the Late Devonian.  All these events appear to have occurred through a freshwater dwelling phase in their evolutionary transition from marine to true terrestrial animals.  The Devonian is believed to have been a time of severe drought, which might have forced these animals (at least hexapods and tetrapods) onto land as their freshwater habitats vanished.

Their manifest destiny assured, insects invaded all the land, air and fresh water niches the continents could provide.  Sounds neat, but being such successful colonizers, why didn’t they ever return to their marine roots?  Simple; all the rooms were taken:

It has been a puzzle as to why hexapods–in particular insects, which possess a morphology that apparently enables them to adapt to virtually all types of terrestrial environments–have not been able to diversify successfully in the marine environment.  It is likewise remarkable that the crustaceans–fulfilling a biological role in the sea comparable to the insects on land–have not been able to invade land to a greater extent despite their considerable age.  The recent phylogenetic analyses of molecular sequence data suggest a paradigm shift concerning the phylogenetic position of hexapods–that crustaceans successfully invaded land as insects.  It is possible that when insects entered terrestrial habitats, their crustacean ancestors had already diversified in marine environments and occupied all potential niches, which could explain why insects were prevented from colonizing the sea subsequently.

So with no room in Neptune’s Inn, they took whatever they could get in caves, swamps, deserts, forests, lakes, high mountain peaks, and suburban kitchens.  Now the Darwinian story holds together again, with an added benefit: “Most important, however, the new molecular results offer a solution to the enigma concerning the absence of marine hexapod remains in the fossil records prior to the Devonian.”

¹Glenner et al, “Evolution: The Origin of Insects,” Science, 22 December 2006: Vol. 314. no. 5807, pp. 1883-1884, DOI: 10.1126/science.1129844.

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    Stay tuned for the next exciting episodes, The Ghost of Kiwi Past and How Rocky Earned His Sails (12/13/2006).