Last week, we watched Evolve: Eyes on the History Channel; tonight, shall we watch the next episode, Evolve: Guts, together? Tune in shortly!
A disgusting beginning: competitive eaters? Bleh. It’s a basic introduction to mammalian digestive physiology — I can tell we’re going to get lots of Big Vertebrate biology again.
They show a cool machine called Cloaca that simulates human digestion, with vats containing chemicals to act as the various chambers. They don’t bother to explain why this machine was built, but it is kind of weirdly interesting.
Once again, they openly say that the complexity of the digestive system evolved — that’s rather refreshing.
Yay! They go back to microorganisms 700 million years ago…for 30 seconds. Then we move on to Ediacaran organisms. The evidence shows that they weren’t photosynthetic, but were some kind of animal that had to have fed somehow, and probably were passive absorbers of drifting nutrients. They suggest that they were replaced by Cambrian organisms that had guts.
Jellyfish just have a sac, not a tube. Cambrian creatures had a more elaborate feeding system, allowing for sophisticated mobile predators, and we see an arms race. Nice animations of Anomalocaris all over the place!
Zip to the modern day: submersibles discover exotic deep sea worms that live on dead whale bones. They have no mouth or gut, so how do they eat bone? They were drilling in and bringing bacteria with them that broke down the bone, and then the worms absorbed the bacteria.
Hey, they mention bacteria, and talk about how digestive enzymes secreted by bacteria are predecessors, and are necessary for the extraction of food in our own guts.
They also mention fish and specialization of regions in the gut tube! More fish, please! But no, we’re going to go straight to tetrapods now, and the promise before the break is dinosaurs. Oh, well.
I’m really pining for more about the actual evolution of guts — and something about development. How can they talk about epithelial tubes without talking about development? Jumping to dinosaurs skips all the interesting stuff. Guts are done by the time you’ve got dinosaurs.
OK, dinosaurs. Yeah, yeah. They discuss gizzard stones and the relationship of dinosaurs to birds. Were dinosaurs warm-blooded? Their digestion was less croc-like and more bird like, determined by analyzing dinosaur coprolites, including T. rex droppings, which contain fragments of fossilized bone. They contain large quantities of bone, which suggests a croc-like eating pattern. But they also contains fragments of fossilized muscle tissue, which suggests that food passed through rapidly, like a bird. So it was a glutton that also had to eat frequently.
The K/T event meant that these big consumers all starved to death. The lead-in to the next section is all about snakes and mammals.
Snakes! Theyre going to talk about the evolution of feeding strategies (why not use cichlids, though? They’d be better). Oh…because you can show movies of snakes swallowing mice whole.
Nifty x-rays of poor mice dissolving in a snake’s gut.
Discussion of the ability of the snake to shut down its gut between meals. Microvilli lining the intestine actually contract while fasting, and increase in length when feeding.
20 million years ago, there was a widespread increase in grasslands that represented a new opportunity … but was hard to eat because much of it was bound up in cellulose. Ruminants evolved fermentation chambers. They show a surgically fistulated cow that allows researchers to get their hands right into a gut. They use bacteria to help break down tough plant material.
These adaptations promote the growth of herbivores…which leads to the evolution of predators.
Now it’s on to humans, of course. They suggest that maybe the key innovation in our ancestors wasn’t our brain, but guts: big-toothed, small-brained apes evolved into small-toothed, big-brained humans. A switch in diet to more meat, and the use of tools to ‘pre-digest’ food allowed us to have smaller guts. Cooking was another huge change that greatly improved the quality of the diet.
They measure the energy required by snakes to digest raw vs. cooked meat. Cooking reduces the cost of digestion by 12.5%. Human guts evolved to be more efficient, liberating more energy for the evolution of the brain.
OK, much like last week’s episode, this show’s strengths are also its weaknesses. The emphasis on charismatic megafauna may be great for catching the attention of casual viewers, but it leaves out all the important events in the evolution of these structures, and ends up emphasizing late refinements and details. Somehow, we need to get a documentary that brings up more molecules and development and the all-important teeny-tiny creatures, where the major innovations first appeared.
But still, I’m most impressed to see a television show that unapologetically discusses evolution as the only credible explanation for the appearance of these features.