When a piece of skin, muscle, bone, nails, or hair gets removed or damaged, those usually grow back like nothing happened. So, why isn’t it possible for hands, or even something smaller like a finger to regrow? Or would a piece of meat just regrow there instead of something useful?
The human body has very limited regrowth potential. It can fix itself to some extent, but only if the injury is at a very small scale and made up of undifferentiated cells (the liver is an exception as it has limited regeneration, the only organ in the body with that ability). So we can fix tiny skin injuries, muscle tears and our bones are fairly good at growing together.
However, if it’s bigger or more complicated than that it’s replaced with scar tissue. We can’t grow new bones, we can’t grow new muscles, we can’t fix large muscle tears or large cuts in the skin (they’re just glued together with scar tissue).
The animals that CAN regenerate limbs (like salamanders, catfish or crustaceans like lobsters and crabs) grow a small blob of cells called a blastema from which they can then sort of replicate the whole limb-growing process. Human’s don’t have that ability.
The body doesn’t know how to build a hand “from scratch” the same way a carpenter knows how to build a house. It only knows how to tell developing tissues to form into many different things, including hands.
Certain genes get activated and certain proteins get made that tell nearby cells what to do.
A tissue on the side of the embryo called “lateral plate mesoderm” starts growing into little buds and forms a substance inside called mesenchyme. This differentiates into cells that will become cartilage and bones, and at a certain stage, parts of it get told to die in order to make the spaces between fingers.
Then these “chunks” of tissue near the midline called somites (paraxial mesoderm) have to form other chunks called hypaxial myotomes, which start this sort of “parade” as they migrate out into the limb and are followed by growing spinal nerves.
During the whole process, lots of developing tissues are all communicating with each other and mutually telling each other what to do. These tissues don’t really exist in the same way in an adult human. Having lots of rapidly dividing and differentiating tissues isn’t the way the human body works.
TLDR: The development of many structures and organs is dependent on contextual signals and communication between developing tissues that only exists in the first few months after fertilization.
That’s the evolutionary lot for many animals and not only humans.
That said, I’ve heard somewhere that there is a tradeoff between complexity and regeneration. The more complex a structure, the more specialized it must be to construct, and the more specialized, the more difficult it is to reproduce that structure once it is lost.
So perhaps, those body parts that can be regenerated are ones that are simple to regenerate. In animals, that would point to it being possible to regenerate tissues, but only rarely organs.
The only part of my answer I am certain is the first paragraph. The rest is my educated speculation.
The way I understand it, and I’m not a biologist, is that we essentially have little templates all around our body. When our nails need a model to make themselves from, they get it from our nail bed, when our bones need that tissue it’s in the marrow for example.
However, all of those are generic structures. You have many bones, many nails, etc. For your entire body like macro structures, it’s much much more expensive to template that and more importantly to keep that specific template alive for your entire lifespan. YES, THAT TEMPLATE, THE BLUEPRINT IS LIVING TOO. YOU NEED TO KEEP IT ALIVE. Remember that. There’s a cost to each part.
Lizards do it over just their tail, but they don’t live long. And every time they do it, its very very energy intensive and takes up crucial nutrients. Things that can regrow limbs like octopi/octopodes (for funsies) are pretty simple structurally compared to us — on a large scale, yes they have very complex inner workings, but the simplicity and redundancy are key here. Same with things like starfish, it’s literally a blob with a shape and organs mixed in.
When we’re first conceived, we’re just a blob too. Our template cells are all there. But as we grow out and get more complex it’s just not worth it for our body to keep those templates alive, it’s pretty much keeping another copy of yourself alive in the background just in case you need it. So our bodies ditch those templates until we’re down to those generic ones I mentioned. The odds of you needing those over time lessen more and more, for example, your bones add density and you build muscle mass.
A child might sustain an injury in ways adults can’t and when they have those template cells they can heal from it and are more likely to survive and procreate. Our bodies toughen up drastically over time. Side note but children are ridiculously durable. That’s a hilarious thing to say but… parents will know lmao
Evolution has a way of trimming down fat. At what point is it more efficient and gives the highest probability of passing on genes vs to just have the individual die but have a healthier more robust offspring by taking out that extra burden of keeping those templates alive.
Why build extra armor or padding if you don’t plan to get hit? If I build a nice fortress and no one will break it, why would I invest in the materials, keep them in storage, and keep those plans. Better to just keep the recipe for a generic brick that you can use to replace the broken ones. And the pestle and mortar to patch it. That’s like your bones. If you lose one, in nature, you’re probably screwed anyway. The odds of you escaping are pretty low at that point. Easier to focus on avoiding that scenario with other means like stronger muscles to fight off or stronger bones. You can see where I’m going.
Now picture those templates as stem cells. Our body tends to really focus on keeping the most generic ones (in the sense of widely used across the body) over the specific yet unlikely to be needed outside of crazy circumstances. That’s why our body doesn’t really keep a lot of heart stem cells, if you get to that point that it’s needed to grow another, you’re probably fucked for some other reason. Like totalling a car… at what point is it more expensive to buy a new one than to add a new engine and in evolutions case… to build that engine and keep it around until you need it. Multiply that for every organ you want to regrow. Zoinks.
The way I’ve seen it explained before, stem cells are like blind construction workers.
If you take a door out of it’s frame, the blind construction worker could feel around and assume it’s a doorframe and eventually install a new one.
If you cut out the doorframe, or take the whole wall, they’re not going to know what was previously there.
Stem cells do kind of the same thing. Small injuries are easy to tell how to fix. Big injuries are less clear how it should be fixed.