In biology, the answer to “how” and “why” are often not very clear-cut.
Size matters.
In biology, size plays a huge role in the relationship of an animal to its environment and to other animals in the food chain. What can strike amazement (and fear) in the imagination are the extremes of size.
Modern insects typically range from barely visible to the size of the palm of our hand. But in the Carboniferous period (of the Paleozoic era) roughly 300 million years ago, there were insects that were larger, much larger.
One kind of insect, Meganeura monyi is type of dragonfly from that era which had wingspan that was ten times the size of a modern dragonfly.
Ever wonder why insects are the size they are today when some of them were the size of sparrows and crows millions of years ago? How did these insects grow to be so big?
The first thing to remember is that there is no way to rewind the tape and replay it again to be sure. Every prediction is based on common sense, fossils we find, and mathematical predictions.
Is it gravity?
The best essay on why animals are the size they are is J.B.S. Haldane’s now classic “On Being The Right Size” written in 1926. There’s a relationship between gravity and animal size, and another between metabolism rate, longevity and maximal size.
But gravity hasn’t changed on the planet so it’s not the reason why one animal would grow or shrink to different maximal sizes on the same planet in different geological time-periods. While gravity might make it harder for larger animals to exist than smaller ones, it doesn’t explain why large animals exist at all.
Is it exoskeletons?
Many insects have hard exterior coverings that can get quite heavy as opposed to bone and cartilage skeletons in larger animals. These “exoskeletons” can pose a constraint on large insects.
This theory explains why most insects are small today, but it doesn’t explain why some were so large before. So, no.
Is it oxygen levels?
One of the most favored theories about insect sizes is that they’re tied to the changes in oxygen in the atmosphere over time. You see, insects don’t have lungs. Instead, they distribute oxygen through to cells through a system of trachea. These openings connect tissues directly with oxygen and depend on diffusion. For oxygen, for example, the greater the concentration in the atmosphere relative to in tissues the faster that it can diffuse.
A large insect needs greater diffusion of atmospheric oxygen to be able to metabolize at its larger body size. There’s a model that predicts that the air in the Carboniferous contained about 35-40% oxygen compared to 21% in the air today. That would’ve allowed for larger insects to develop.
In fact, in laboratories scientists have taken modern insects and have grown them in higher concentrations of oxygen and they’ve often come out bigger.
While most scientists believe that higher oxygen makes it possible for larger insects to exist, it doesn’t cover all the bases either. And there are two major flaws to this idea.
It’s true that some dragonflies were huge. But some weren’t. From the fossil record we also know that there was some smaller insects too so #NotAllBugs.
If oxygen levels were the sole determinant of maximal insect size then they should track together. If you remember Jurassic Park, you’ll remember that during the age of dinosaurs there were dragonflies that were twice the size of the largest ones today even though the oxygen levels were around half.
It turns out that we don’t really think that all insects simply respire by diffusion. Some beetles have thoracic pumps that let them grow larger than one might predict simply from oxygen levels.
Is it oxygen toxicity?
By now I should have you convinced that more oxygen allows for at least some larger insects. But what if I told you that large quantities of oxygen are also poisonous to insect larvae?
That’s right there’s another theory that insects grew large because their larvae were large. And larvae had to grow large to absorb less oxygen. A larger organism has less surface area compared to its volume.
But again, this theory doesn’t explain why some insects were small at all during the Carboniferous.
Is it birds?
A few years ago, an American scientist, Matthew Clapham published a highly controversial paper that suggested that giant insects disappeared around the time that birds evolved.
The idea goes something like this. Large insects like massive dragonflies ate smaller insects. But once bird which were faster and more efficient predators of small insects evolved around 150 million years ago that ecological space was taken up.
Birds have been eating up and keeping insects small ever since.
So, what is it?
It could be any of these reasons or a combination of these reasons for different insects during different time periods. We may also discover new fossils or evidence that changes what we know.
Sometimes the answer is in genes, sometimes, the answer is in the environment, and sometimes we have to humbly admit that we just don’t know. Science is exciting but it is always in a state of flux.
Life is messy, so why should its study be any different?
Long COVID and brain damage.
I spent time looking into one worrying aspect of COVID-19- damage to the brain that lingers for months after the virus is no longer be detected for my column in Hindustan Times this week.
A few key points (you can read the entire piece here; click “skip” or register).
We don’t know yet if COVID-19 causes permanent brain damage or dementia, but disturbing signs called "Pre-Alzheimer's" have been seen in some. There are abnormalities in the brains of those with some symptoms of Long COVID.
Alzheimer’s is characterized by irreversible long-term memory loss. It is detectable in the clinic; tell-tale signs are plaques and tangles of misfolded proteins. There are many reasons why it develops, and increasingly infections are thought to play a part in some people.
MERS, SARS, Zika, and Ebola are also linked to brain disorders. Herpes virus has been found in brains of those with Alzheimer's (which is permanent). Major neurological and psychiatric disorders happened after the H1N1 influenza pandemic of 1918-1919 too.
What is worrying for COVID-19 is the number of people infected globally which is in the 100s of millions. COVID-related brain disorders have pushed some experts in favor of the second booster (third dose) even though the true benefit isn't known.
If you’ve had COVID-19 and are suffering from “brain fog”, loss of smell or taste, reduced vision, post-traumatic stress disorder, or depression weeks after getting a negative PCR test, see a doctor. There is a brain study currently underway (Indian doctors are part of it).
If you're sitting on the fence on vaccination because you're healthy, and your risk of death is low, WHAT ARE YOU WAITING FOR? Get vaccinated. Long COVID is serious, and potentially life changing.