Animals do the most amazing things. Read about them in this series by Janaki Lenin.
In the race to conquer the skies, insects beat vertebrates by at least 90 million years. At one time in their evolutionary history, they were incredibly rare, leaving no trace in fossil records.
A team from Stanford University and a clutch of other institutions investigated why bugs were so scarce. To understand how elusive the answer is, consider that insects are the no. 1 life form on Earth today, occupying every niche and continent. More than 75% of all known animals belong to this group. Just ants and termites alone make up 20% of the world’s animal biomass.
The oldest fossil of an insect is a wingless creature about 385 million years old. After that brief appearance, they seem to vanish for 60 million years, a period called the Hexapod Gap. Then something happened to encourage insects to multiply. Why are fossils of bugs so rare during the gap?
One theory proposed that insects didn’t get enough oxygen. Another suggested that they may have been abundant, but the geochemical conditions were not ideal for fossilisation, which gives the impression of insect scarcity.
In 2006, Robert Berner, a geologist at Yale University, came up with a model to reconstruct the ratio of atmospheric oxygen prevalent over the past 570 million years. At one time during the Hexapod Gap, atmospheric oxygen was less than 15%, so low that even wildfires couldn’t have burned. Perhaps insects didn’t do well in this low-oxygen air.
Sea levels also rose twice during the gap, which could have prevented fossils from forming. So perhaps they didn’t preserve at all.
“The Hexapod Gap is one of the outstanding mysteries in the history of insect evolution,” says doctoral student Sandra Schachat from Stanford. The last time anyone studied this period was a decade ago. “During the past ten years, a lot of progress has been made in understanding Romer’s Gap, which is a similar gap in the fossil record of early amphibians. My collaborators and I were inspired by this progress, so we wanted to see whether we could make progress in understanding early insects.”
When Schachat and her team examined updated stable isotope data, the level of atmospheric oxygen wasn’t as extreme as Berner had estimated. They say that throughout the Hexapod Gap, atmospheric oxygen never dropped below 15%. Widespread charcoal deposits also support this conclusion. Besides, insects alone aren’t likely to be affected by low oxygen when their close relatives, the arachnids (spiders and scorpions) and myriapods (centipedes and millipedes) that colonised land before them seemed to flourish.
The researchers then looked at different types of ancient rocks in the Macrostrat database, a collaborative platform to explore geology. There was no shortage of terrestrial sediments. If there was enough oxygen to support insect life and the sedimentary foil to preserve it, what explains the lack of fossils?
Schachat and her team examined the Palaeobiology Database, a public resource to explore fossil data. “The Hexapod Gap appears to be the tail end of a longer interval in which insects were rare compared to arachnids, centipedes and millipedes because insects had not yet evolved wings,” says Schachat.
This led them to conclude that, had there been insects, they would have fossilised.
One main difference between the two extremes of insect rarity and abundance is wings. As soon as fossils of the first winged insects emerged, they rapidly outnumbered arachnids and myriapods. By taking to the skies, they could escape predators and go where there was plenty of food. Wings liberated insects to colonise new niches and habitats, say the researchers.
“This is a fantastic paper, done by a collaborative group of scientists,” Roy Plotnick, professor and palaeoentomologist at the University of Illinois, Chicago, told The Wire. “This study exemplifies how paleontological research is currently conducted – utilising data and methods from a diverse group of fields. The most important part of this paper is the reanalysis of oxygen levels in this interval to show that they were higher than originally thought and that later levels were not as high. I am in overall agreement with their assessment of the reason for the lack of insect fossils prior to the evolution of wings.” Plotnick wasn’t involved in the study.
A similar lack of fossils, called Romer’s Gap, plagued early vertebrates. However, recent fossil discoveries in Scotland showed this gap was an illusion and not a result of few animals or even fossils. Experts were not looking for them in the right place.
Could the same issue cause the Hexapod Gap?
“Many people have tried to discover insects from the Hexapod Gap, though publications about ‘insects’ from this interval are usually challenged afterwards,” replies Schachat. “There are a few cases in which things like crustaceans or centipedes appear to have been mistaken for insects. Palaeontologists really have put a lot of effort into discovering insects from the Hexapod Gap, so we have no reason to think that real insects have been systematically overlooked.”
Plotnick says wings preserve better and comprise the majority of ancient insect fossils. Lake sediments make the best substrate for fossilisation, and there were few such locations.
“It is reasonable to assume that the earliest insects were quite tiny, which would also have made preservation and collecting difficult,” he says. “If there is one factor [the researchers] omitted, it is the possible role of cuticle structure in the lack of fossilisation.” The exoskeletons of scorpions preserve well and are often the only arthropods found in many terrestrial fossil deposits.
Jennifer Clack, the senior member of the team that explained Romer’s Gap and professor emeritus at University Museum of Zoology, Cambridge, also agrees with Schachat and her team. “Over the last six years, we have worked on a project specifically targeting continental rock sequences, including lake and river deposits in the UK and North America,” says Clack.
She studied a site called East Kirkton, near Bathgate in Scotland, for several decades. “The preservation of animals and plants there represent the earliest terrestrial assemblage so far known, and it includes many arthropods – scorpions, eurypterids [sea scorpions], a harvestman, and millipedes. They are preserved in volcaniclastic shales and tuffs in a very unusual environment that preserves arthropod chitin well. We really expected that this place would reveal winged insects. But no luck. So, I am inclined to agree with the authors, that they were rare or highly inconspicuous.”
Despite the lack of fossil discoveries, Clack thinks not looking in the right spots and the low proportion of continental sequences are also factors. “Insects should certainly be there, even if not yet winged,” she says. “My guess would be that they were freshwater organisms, given the preponderance of aquatic larvae in the most primitive insects today.”
If the lack of early insect fossils is a mystery, the mechanics of how they became abundant is the subject of debate too.
But what is the origin of the insect wing?
This question, one of the most contentious in evolutionary biology, nagged entomologists for over a century. Fossils of insects with proto-wings, illustrating the transition from wingless to winged creatures, would have been useful. But they’ve had to grapple with the Hexapod Gap.
One, the tergal hypothesis, proposes flat wings first erupted from the upper side of the thorax, enabling bugs to glide at first. But once they began flapping them, they needed muscles and flexibility of movement that tissues of this area could not provide.
The other theory – the pleural – proposes tissues related to legs moved upwards and expanded, so wings budded from the sides of the body wall. The muscles of this area already powered the legs and could pump the wings. But could it provide the structure to the wing? Each theory had something going for it.
Some think these two theories don’t necessarily exclude one another. Instead, they argue that both are valid. When flying insects metamorphose into adults, the gene network in the tergal and pleural tissues probably fused to form a composite wing. The tergal plate could provide the wing structure and the pleural plate, its musculature.
Insects didn’t make the compromise that vertebrates would later make: lose forelimbs to gain a pair of wings. Instead, they grew their flying appendages and retained all three pairs of legs. The thorax, between the head and abdomen, has three segments, with a pair of legs sprouting out of each. In flying species, wings grow only on the second and third segments.
These segments have a gene that codes for wings. In the red flour beetle (Tribolium castaneum), the researchers from Miami University found the gene in the first segment that doesn’t sprout wings. What prevents the creature from growing three pairs of wings? A regulating gene suppresses the growth of wings in this segment. When the researchers interfered with its functioning, the tergal and pleural plates fused to grow a full wing in the first segment. Could the world’s first wings have grown in the same manner?
Even insect abdomens have this wing gene although they sprout neither legs nor wings. Instead, it governs the growth of a defensive structure called the gin-trap in the pupal stage. Pupae are vulnerable targets for predators. When anything touches them, these mouth-like traps close and open rapidly enough to threaten or even pinch their aggressors.
How does studying a living species show how the proto-wing of an insect ancestor may have evolved?
The archetypal insect body structure has uniform segments arranged sequentially. For example, the three thoracic segments that grow legs. Evolution repurposes such similar structures for other uses, like the grasping front legs of praying mantises. Similarly, the abdominal segments of the flour beetle modified the wing gene to create the gin-trap. But these repeated units may also retain their original purpose. If the researchers can reset these genes to produce what they had initially evolved to do, then they would be afforded a sneak peek into how evolution may have worked.
Studying real wings doesn’t help researchers answer their evolutionary origin.
“One key concept here is that we can’t readily study this phenomenon in the typical wings of insects because these wings have already evolved and thus the fusion of these two tissues has already occurred,” explained David Linz, the main author of the paper that describes the study.
To this end, the researchers used a transgenic line of beetles that glow a fluorescent green wherever a wing gene called nubbin exists.
“Normally, this is just in their actual wings as nubbin is a really ‘classic’ wing gene,” Linz said. “So we use nubbin because it can act as a marker for tissues that have wing identity. In the abdomen of these beetles, there usually isn’t any green because there are no ‘true’ wings there.” Linz, formerly with Miami University, is now a postdoctoral fellow at Indiana University.
The abdA gene suppresses the growth of limbs in the abdomen. When the researchers neutralised that gene, “all of a sudden, we saw these green spots,” says Linz. “Meaning, in a sense, we had wings along the abdomen where they shouldn’t be!”
Achieving this was by no means an easy task. “We spent a long time ‘dialling in’ the precise conditions necessary for our gene manipulations to obtain the animals with the perfect transformations,” he said. “I manipulated nearly a thousand beetles to determine when, and by how much, to tweak these genes to get the ideal intermediate transformation. But once these parameters were fine-tuned, we saw the nice gradient of transformation that you can see in the second image. It was really exciting!”
To prove that both dorsal and pleural tissues together form wings, the researchers suppressed abdA as well as another one, a Hox gene called Ultrabithorax (Ubx). Hox genes regulate the functioning of other genes according to the body plan – directing, for instance, when and where limbs should grow.
“When we did this, we saw green dots form in one location corresponding to the dorsal tissue, and another location in a lateral position,” says Linz. “We deciphered what tissues contribute to ectopic [abnormal] wings in the abdomen.”
They saw two separate tissues merge to form these unusual wings. “This allowed us to support this dual origin of insect wings,” he said.
“This paper is interesting because it provided new evidence for the dual-wing origin hypothesis,” says Shigeo Hayashi, of the RIKEN Centre for Developmental Biology, Japan, who wasn’t involved in this study. By comparing genes related to wings in primitive insects such as bristletails and mayflies, Hayashi’s team published one of the earliest papers to support this hypothesis.
But Linz cautions against drawing any sweeping conclusions.
“Strange things can happen during long evolutionary time periods and using only a single representative species to interpret the past can point us in confusing directions,” he says. To further investigate the origin of the insect wing, he suggests similar evolutionary development studies in more insect species as well as their close relatives, crustaceans. It is equally important that other fields like palaeontology or morphology contribute to answering this question, he says.
Although the insect family is large with a variety of wing shapes, forms, and colours, the flying appendages evolved only once in its lineage. But despite insects’ phenomenal innovation and success, scientists estimate human activities have reduced flying insects by 75% in Germany.
Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.