Evolution's Hidden Wildcard: The Single-Cell Bottleneck

imagine you're surprised if while flying a kite on a warm spring afternoon a sudden gust of wind ripped the string from your hand slicing off your pinky finger gross now imagine your surprise of several days later you started growing a new finger and your severed finger started growing a new you I'm sure this would be front page news all around the world but why the normal way that humans and many other animals reproduce is actually more impressive you started out your life as a single cell once that cell was fertilized it started copying its DNA separating those copies and then splitting in two two became four four became eight until eventually there were trillions each doing their own little part to make you who you are today if a single cell can grow into a full-fledged human why can't a finger after all the chance that a single cell will die or possess some sort of genetic defect is huge but a finger contains millions of cells if one or many cells in that finger are defective the others would be there to take over right at first glance this seems like a better way to reproduce and it turns out that there are plants and some animals who do reproduce this way biologists call it budding Hydra for example our little multi-celled animals found in pond scum one of the ways they reproduce is by simply sprouting and then shedding a little copy of themselves after having a good meal some species of flatworm are able to reproduce through fission just like us they are multi-celled animals just like us each of their cells has its own entire copy of the organism's DNA just like us different cell types skin cells versus stomach cells for example are produced by turning specific genes off and others on within those cells but unlike us when these animals want to reproduce if they can't find the mate they simply rip themselves in half both chunks regenerate any missing body parts and voila reproduction has occurred if this is possible why is it that in humans and so many other animals Evolution bets it all on one single cell every generation is this an example of Reckless gambling or did the blind process of evolution actually stumble upon a very brilliant strategy stated clearly presents Evolutions hidden Wild Card the Single Cell bottleneck when biologists talk about the Single Cell bottleneck they're referring to the strange fact that so many multi-celled organisms start out as a single cell in sexual reproducers of course that single cell the zygote is a fusion between what you could call half cells most commonly a sperm cell and an egg cell from each parent pretty much any animal you'd find at a zoo so all vertebrates from tiny fish to giant elephants all insects and even cephalopods squid and octopus all of them use a single cell bottleneck every time they reproduce even many species of flatworm Hydra and other animals that can reproduce through budding and fission they will occasionally mate and use a single cell bottleneck instead why restarting each generation is a single cell that's an extremely complex extremely risky process whenever we find a complex trait like this in biology this usually means the trait is doing something important if it wasn't the Relentless filter of natural selection likely would have stopped the process or trait from ever evolving so what does the Single Cell bottleneck do for us what would be your guess one thing it does for us is it corrects any defects that an adult might have defects that were caused by bad conditions during development Charlie here has great genetics his DNA recipe seems to be nearly Flawless but while developing in the womb his umbilical cord happened to wrap around his hind leg dramatically stunting its growth by the time he was born the cake had been baked he now must live his entire life as a tripod if the ever father's kittens that developmental error will be automatically corrected by the Single Cell bottleneck those Offspring will have all four legs likewise a fresh start can easily correct any injuries and epigenetic glitches that might accumulate through an animal's lifetime errors that are too complex to fully heal or reverse while an animal is actively trying to live its life most importantly the Single Cell bottleneck resets a state that scientists call genetic clonality what is genetic tonality and why does it matter if you've ever studied Wildlife Conservation you probably know that at the population level genetic diversity is key to the survival of a group of animals when each individual has a unique DNA sequence resulting in different traits this ensures that at least some animals in the group are likely to survive no matter what challenge comes their way but in many cases the opposite is true when you compare DNA sequences between cells of a single individual at this layer of organization too much genetic diversity can actually lead to problems the behavior of each cell in your body is influenced by many things including conditions in the environment around you chemicals and even parasites that might get inside you signals from neighboring cells and the influence of epigenetic markers but at its core a cell's behavior depends on its DNA sequence as cells replicate mutations can occur in DNA some of which can change how immune mutated cell will behave two cells are considered perfect genetic clones if they have identical DNA sequences early on in development all or most of your cells were perfect genetic clones of the original but by the time you're an adult mutations accumulate meaning the cells of your left hand have slightly different DNA sequences than the cells of your right hand some genetic differences can cause conflict between cells one extreme example would be a lung cell that mutates in a way causing it to rapidly reproduce out of control in most cases neighboring cells will eventually stop this from continuing but if they fail cancer is born a less dramatic example would be any mutation that simply causes a cell to misbehave liver cells perform a very dangerous job they help filter toxins from your blood conditions can be so harsh inside the liver that it's common for a liver cell to die and have to be replaced before the cell is even one year old knowing this imagine a mutation in one of your liver cells that causes that cell to stop helping filter toxins this might allow that single cell to live a longer healthier life than its neighbors but now it's just kind of taking up space and using resources without providing any service in return the cell Colony as a whole in this example that colony is you will be slightly weaker as a result it turns out that our bodies go to drastic measures to stop internal conflicts between cells in extreme cases immune cells will seek out and Destroy any individuals behaving as if their DNA has mutated too far from the norm an easier far less violent way to ensure that each cell shares the same DNA is to start each generation through a single cell bottleneck even though it might seem wild the gamble all hope on a single cell the rewards often do outweigh the risk if that single cell happens to have a negative mutation the new organism will often fail to develop early on allowing the parents to simply try again on the other hand if that single cell happens to have a beneficial mutation that beneficial mutation will end up in every single one of the new organism's cells 2. hedge their bets even further in many animal species individuals carefully choose who they'll mate with they tend to prefer Partners who are healthy or who are otherwise doing well in the struggle for existence on top of this each male and many species produces and releases millions of sperm at a time that must race each other to fertilize a single egg cell the genes inside each particular sperm cell are the genes that built that particular sperm cell this means a competition between sperm is a competition between genes only those containing the best cell building genes are likely to fertilize the egg biologists who study the mathematics of natural selection have found that single cell bottlenecks are so useful it's not clear how some animals get by without them a collaborative project between scientists at Oxford Stanford and the Mac Planck Institute of guttingen are currently studying a population of flatworms that so far as we can tell never use single cell bottlenecks they strictly reproduce through fission is this population doomed to slowly degrade through mutational meltdown or have flatworms evolved ways to effectively work with or even benefit from genetically diverse cells finding the answers to Mysteries like this not only helps us better understand the world around us but could lead to breakthroughs in the treatment of things like cancer and aging so to recap here at first glance reproducing through a single cell bottleneck seems like a dangerous wild gamble but it actually does many things for us among them it prevents conflicts from popping up between cells by resetting genetic clonality every generation this reset is so essential scientists are currently trying to understand how some flatworms and other animal populations seem to survive without it I am John Perry and that is the Single Cell bottleneck stated clearly this video was funded in part by a grant through the Volkswagen Foundation awarded to Ashley Griffin Johan rink and Beau Wong it's part of Volkswagen's research program called life which aims for a fresh scientific approach to the basic principles of life if you want to learn more about single cell bottlenecks and in particular the strange organisms that don't use them check out the Articles linked down in the video description this animation was also funded by our viewers on patreon.com forward slash stated clearly

Channel: Stated Clearly

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Length: 11min 8sec (668 seconds)

Published: Fri Jun 02 2023