There are some situations in life where it’s probably better to be big -- like if you’re
a rugby player, or a piano mover, or a sumo wrestler.
But size can have an evolutionary advantage, as well.
In the animal kingdom, a bigger body size, or biovolume, has often been found to go along
with greater biological fitness -- that is, the ability to thrive and reproduce.
This has led many biologists to speculate that, over time, natural selection has favored
larger animals, and that animals have probably been increasing in size.
It’s known as Cope’s Rule, after American paleontologist Edward Drinker Cope, who noticed
in his study of fossils that some organisms -- especially mammals -- seemed to increase
in biovolume over evolutionary time.
But Cope’s Rule has been seen as more of a gut feeling, generally unsupported by scientific
research…
until now.
In an article published this week in the journal Science, researchers from Stanford University
found that marine animals, at least, have definitely been getting bigger -- their biovolume
has increased by an average of one-hundred-fifty times over the past 542 million years.
Now, you’d think that noticing whether or not animals have gotten bigger would be pretty
easy.
But it turns out to be anything but.
The scientists compiled a massive database of body measurements from five phyla and 17,208
genera of marine animals.
Their data ran from the present all the way back to the Cambrian Explosion -- the most
biologically productive period in Earth’s history, starting nearly 550 million years
ago, when about half of the animal phyla that exist today first started to appear.
The scientists took all that data and entered it into a computer model, and a pattern emerged.
First, they found that the minimum size of marine creatures decreased by a factor of
10, the maximum size exploded 100,000 times during the same period.
Also, they also found that the groups of animals that saw the greatest increase in biovolume
also tended to become more genetically diverse.
What this means, they think, is that larger animals are more likely to turn into more,
different species than smaller animals, because their increased size meant they can travel
farther.
This allowed them to expand their home ranges, and diverge into different populations, which
are the building blocks for the creation of new species.
But what exactly is driving this selection for bigness still isn’t clear -- and the
researchers also can’t explain why it seems to apply to marine animals and not other kinds,
like insects.
Speaking for myself, though, I’m perfectly happy with bugs being the size they are right
now, thanks.
Also in the news this week: New insights into one of the most medically useful effects of
cannabis.
Cannabis is being prescribed more and more as a medical treatment -- often as an appetite
stimulant.
When ingested, it can induce an almost insatiable urge to eat, which can be helpful for people
with suppressed appetites, like cancer patients undergoing chemotherapy, or patients with
HIV or disorders of the nervous system.
How exactly the drug creates this effect has been unclear.
But according to new research, cannabis seems to stimulate the appetite by basically confusing
the heck out of the brain’s food center -- in the hypothalamus.
Scientists from Yale University reported this week that the sudden hunger caused by cannabis
is actually driven by neurons that are normally responsible for suppressing appetite.
See, many of your brain cells are covered in receptors that allow them to communicate
with each other by chemical signals called neurotransmitters.
And some of these neurotransmitters are cannabinoids -- chemicals that, when accepted by your neurons,
can inhibit the release of other neurotransmitters.
The active ingredient in cannabis -- known as THC -- basically imitates a cannabinoid
and binds to its receptors, interfering with the normal function of neurons.
Now, among the brain cells that have cannabinoid receptors on them are a special set of cells
that are thought to regulate eating -- known as POMC neurons.
And it’s thought that these brain cells stop us from eating too much, by transmitting
signals that give us the sensation of being full.
So... when the scientists placed well-fed mice in front of a bowl of food and injected
similar chemical to THC into their brains, they expected that their hunger-neurons would
become less active.
But instead, the drug basically threw them into overdrive ... and reverse.
The POMC neurons in the mice stayed active, but their activity actually switched on the
hunger circuitry inside the hypothalamus.
So instead of not eating, the mice in the experiment began to eat, and eat, and eat,
despite the fact that they had already been fed, unlike mice who had not been given the
drug.
Now, why THC can flip these neurons into reverse is still unclear.
The scientists say they have to do more research to confirm what they’ve seen.
But their findings may be useful not only in understanding how cannabis creates this
useful side effect, it can also help our basic understanding of how brains control -- or
sometimes fail to control -- our relationship with food.
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