During the Paleozoic era, the Earth teemed with giant insects, from dragonflies the size of seagulls, spiders as wide as tires, to mayflies nearly 45 cm in breadth. Today truly giant insects no longer exist. Why did giant insects live in prehistoric times, but disappear from the Earth over time? Why are insects smaller now?
The answer is: oxygen.
-- oxygen history
Most of the oxygen in the air right now comes from photosynthesizing plants and bacteria; but how did organisms start producing oxygen in the first place? The first living organisms used sulfur to drive their metabolism. We call such creatures anaerobic bacteria, and similar microbes still exist today (responsible for the smelly gases we associate with a rotten flesh coming from decomposing body). However, about 3 billion years ago, one line of anaerobic bacteria evolved to cyanobacteria, which power themselves with sunlight. The process of converting sunlight to useful energy starts with chlorophyll that acts like a biological antenna and absorbs light from the sky. Basically, cyanobacteria use sunlight to break certain molecules down and then create other molecules to store energy for later use. For example cyanobacteria might break water into H2 and O2, then fuse some of those fragments with CO2 and other molecules to make sugars like glucose. This process called photosynthesis produce free oxygen gas O2 as a by-product**.
** getting a bit deeper here (optional read): It takes 6 molecules of carbon dioxide and 6 molecules of water to make 1 molecule of glucose as what we have learned in high school: 6CO2 + 6H2O --> C6H12O6. But notice that the sugar contains only 6 oxygens. It contains only 6 out of 18 oxygens available at the beginning. Conservation of mass says that atoms can neither be created nor destroyed so we can conclude from this that the photosynthesis must produce oxygen gas as its by-product.
As cyanobacteria began to thrive and spread, free oxygen also began to accumulate for the first time in Earth's history. Free oxygen seems nice today, but back then the organisms found it toxic because when ultraviolet light strikes on oxygen, the gas mutates and forms free radicals that chew through DNA and proteins and shred them. Oxygen was profoundly antilife back then (search for Oxygen Catastrophe or Great Oxidation Event to know more). But life on Earth pulled through. Some developed tougher outer membranes to keep oxygen out, some built special interior walls to shield delicate molecules, and some exploit this gas for energy when oxygen rushed through their membranes (search mitochondria to learn why we need oxygen to live).
Oxygen now makes up 21 percent of our air. But, it didn't accumulate steadily over the billennia; it spurted. The first spurt was 2.3 billion years ago during the Great Oxidation Event. Starting 1.8 billion years ago, oxygen level paused and held steadily for a while but started creeping up again about 600 million years ago. In the hundreds of millions of years since, oxygen levels have veered drunkenly, dipping as low as 15 percent and rising as high as 35 percent.
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| Geological time scale |
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| Atmospheric oxygen levels over geological time scale |
-- oxygen and insects
Of all animals, insects probably benefited the most from high oxygen levels. Because insects lack lungs, they can't really inhale oxygen; it passively diffuses into their cells instead through pores in their exoskeletons. Insects take in atmospheric oxygen through spiracles, openings in the cuticle through which gasses enter and exit the body. Oxygen molecules travel via the tracheal system. Each tracheal tube ends with a tracheole, where the oxygen dissolves into the tracheole fluid. The O2 then diffuses into the cells.
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| insect respiratory system |
When oxygen levels were higher (hyperoxic) -- as in part of Paleozoic era -- this diffusion-limited respiratory system could supply sufficient oxygen to meet the metabolic needs of a larger insect. Oxygen could reach cells deep within the insect's body, even when that insect measured several feet long. As atmospheric oxygen decreased over evolutionary time, these innermost cells could not be adequately supplied with oxygen. Smaller insects were better equipped to function in a hypoxic environment. And so, insects evolved into smaller versions of their prehistoric ancestors. It's a geometric fact that surface area increases more slowly than volume so at some point, a size is too large that their little pores (spiracles) can't suck down enough oxygen for their volume. Back in the heady days of 35% oxygen that wasn't a constraint but with 21% oxygen now the insects are tiny because they will suffocate otherwise.
Interestingly though, around the end of the Jurassic and beginning of the Cretaceous period, about 150 million years ago, all of a sudden oxygen goes up (see graph) but insect size goes down. Why? Shouldn't the size increase? Turns out that there is another limiting factor: birds evolution. With predatory birds on the wing, the need for maneuverability became a driving force in the evolution of flying insects, favoring smaller body size. The authors that proposed this theory, Chapman and Karr are unclear whether birds preyed upon the giant insects or simply out competed them, but what is clear is that the oxygen level in the atmosphere is not the only factor influencing the size of insects. There are environmental factors too.
