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Not all hypotheses need to be good. In fact, many of them are terrible. It’s just that when you’re trying to understand the world, you might find yourself believing that there are tiny humans living inside the heads of sperm, and we're here to tell you, that's not how it works.

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This episode is sponsored by Fabulous, an app  that helps you form healthy habits that stick.   Click the link in description to get a free  week trial and 25% off a Fabulous subscription!  This is not an organism.

At least, it’s not  yet anyway. What we are looking at are a lot of   zoospores packed together, getting ready  to be released so that they can find   the right place to grow into themselves.

And this? Well, this is a male tardigrade   filled from head to many toes  with sperm. Just chock full of it.  We did not start this channel with the intent  of becoming so familiar with tardigrade mating   habits, but tardigrades are easy to find, and  their mating habits are straightforward enough   to track, so we keep getting drawn back into the  details and the drama of tardigrade reproduction.  In the past, we’ve talked about the particulars  of their mating habits.

And today, we’re going to   use this tardigrade as a convenient starting point  to talk about the history of our understanding of   sperm. Because as absurd as it might seem to  be looking at tardigrade sperm, the theories   people have had about sperm are much weirder. Humans have been theorizing about the contents of seminal fluid long before the invention  of the microscope.

We did not, as a species, need to work out all of the scientific mechanisms  underlying sexual reproduction to make babies. But the observation of children who carried traits  from their parents naturally raised questions   about how those traits were passed on. And those questions inevitably led   to some hypotheses.

And look, not all hypotheses   need to be good. In fact, many of them are  terrible. It’s just that when you’re trying to   understand the world, you need to start somewhere,  and as long as you’re willing to work with your   ideas and challenge them, then hopefully some  day, you go from somewhere, to somewhere better.  Some of these early ideas about sperm weren’t just  bad though, they were, maybe as we should expect,   a little self-serving to the male ego.  Masturbatory, even, you could say.   Like the theory now known as spermism, advocated  for by one Pythagorus, yes, that Pythagorus.  According to spermism, the information driving  heredity came entirely through the semen.   Whether that was the color of someone’s hair  or their height or whatever other trait—   all of that came through the father, and the  father’s father, and so on.

The mother’s role   was just to provide nourishment to the sperm  as it developed and manifested those traits.  It’s easy enough to find flaws with this idea.  After all, our observed reality, which Pythagorus   just chose to ignore, shows us that traits pass  from mother and father alike. But this would not   be the last theory to overestimate the sperm. Even with the advance of scientific tools over   the ages, there was plenty to get wrong.

One  of those key scientific tools was, of course,   the microscope, which allowed scientists to  observe the cells within semen for the first time.  But just because microscopists could see sperm  now, that did not mean they could make out the   details. And this invited speculation, including  the idea that we are essentially developed from   a much smaller version of ourselves.  This theory was called preformationism.  At the end of the 17th century, the Dutch  scientist Nicolaas Hartsoeker sketched out   his spermist vision of preformationism: inside the  head of the sperm cell, he speculated, was a very,   very, very small human--later called a  homunculus--just waiting for a womb to grow in.  And look, we have the advantage of centuries  of advancements to know that the homunculus   was wrong. But maybe take a moment to think  about how weird that would be if it were right.  Look at this tardigrade, full of sperm.

And  then just imagine if we zoomed in further on   those sperm cells, and we could see little  tiny tardigrades inside of them. And then,   if we could zoom in even more on those  tinier tardigrades, we’d see even more tinier   tardigrades inside of them, and so on and so on. Now, to be clear, that is not what tardigrade   sperm looks like.

Their sperm is made up of a  slightly globular head connected to a flagellum.  But as wrong as both spermism and tiny people  inside of sperm turned out to be, they’re not that   far off the mark if you think about the zoospores  we showed at the beginning of the episode.  They are a tool of asexual reproduction in a  lot of different organisms, including algae,   fungi, and bacteria. They all  use zoospores in different ways.   And because we don’t know exactly what organism  these zoospores come from, there are specific   details of their lives we just don’t know. But take, for example, the Phytophthora,   a group of oomycetes most notorious for  their role in the 19th century potato blight   that led to the Great Famine in Ireland.

Phytophthora can reproduce sexually, but they mostly rely   on making asexual zoospores, which pack together  in a biological enclosure called the sporangia   until the conditions are right for their release. And when they emerge from the sporangia,   the zoospores start swimming, using  their flagella and chemical sensors   to find the ideal place to settle and germinate. All of the hereditary information in those   zoospores came from one parent, fashioning an  organism that resembles the infant form of that   parent, and that seeks only nourishment  from its new host.

The outlines of that   process sound pretty similar to how early  philosophers and scientists envisioned their   sperm-centric ideas of sexual reproduction. But what those scientists missed—what   separates the sperm from the zoospore—is  the nature of sexual reproduction itself,   of the genetic variation it drives by combining  and recombining traits from different parents.  Sperm cannot accomplish that on their  own. Sperm needs an ovum to fertilize,   which scientists would eventually uncover in  the 19th century with experiments on rabbits,   sea urchins, and starfish eggs that helped to  further our understanding of the molecular and   cellular collaboration that set the stage for  inheritance.

Yes, my friends, as Napoleon drove   to conquer Europe, we had no idea that sperm and  eggs came together to make the next generation.  There are a lot of different types of  spores out there, used for both sexual   and asexual reproduction. And in contrast,  sperm might seem more narrow and focused,   both in diversity and purpose. But the function of sperm is so specialized   that it requires its own diversity.

Even among  tardigrades, there are different morphologies to   the head and middle structures of the sperm, small  differences that distinguish between species.  And that’s because however weird  our theories about sperm have been,   the reality is also very strange. Of  all the cells in the body, sperm cells   are maybe the only ones that are expected to  navigate and survive in two different beings.  The homunculus was wrong, but the idea of there  being an image of our life passed on within   sperm and in spores, it isn’t absurd  at all. It’s just too literal.

Instead,   that image is abstracted and tucked away into  genetic information. And zoospores and sperm   alike are just packets of that information,  swimming off to new and different futures.  Thank you for coming on this journey with us as  we explore the unseen world that surrounds us.  And thank you to Fabulous  for sponsoring this episode.  Fabulous is an app that was born at Duke  University and uses behavioral science   research to gently support your personal goals. Habit changing and habit building is hard.   So, if you’re looking for ways to add a new  thing to your routine, Fabulous is the way to go.  It’s the #1 self care and habit forming app  in the app store with over 20 million users,   and it can help you whether you’re  looking for ways to stay focused at work   or you just need a reminder to take a break  once in a while to stretch and drink some water.  The app is 100% personalized to your goals and you  can start building your ideal daily routine today!   The first 100 people who click on the link  in the description will get a free week   trial and 25% off a Fabulous subscription!

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