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.    In the meantime, thanks for watching SciShow News. If you want to keep getting smarter with us, you can go to YouTube.com/SciShow and subscribe.