The third episode of Wonders of Life had the theory of natural selection as its theme, but once again didn’t approach it from the direction I expected. Instead Cox started by talking about how the most important element for life is carbon, because of its versatile chemical properties that allow it to form large & complex molecules with a variety of other elements. These molecules include proteins (which are the building blocks of organisms) and DNA (the instruction set). So he started by telling us about carbon being formed in stars, and then talked about how carbon in the atmosphere gets into organisms. The first stage is photosynthesis – where plants take CO2 and energy from the sun and turn them into sugar (a molecule with a carbon backbone) and O2. From here Cox moved on to talk about how the carbon that the plants are made up of move through the food chain – a lot of animals eat plants, but they are hard to digest because a lot of the carbon is bound up in molecules like cellulose & lignin which are important structural parts of plants. Termites solve this problem by farming fungus in their colonies, which digests the wood they bring it and then the termites eat the fungus. Giraffes in common with other ruminants have a complicated digestive system with multiple stomachs, one of which contains bacteria which help break down cellulose. Other animals take the shortcut of eating animals instead of plants – there was some great footage here of a leopard coming to pay a visit to the (very open!) car that Cox & camera crew were sitting in. I don’t think I’d want to go on safari, that’d freak me out!
Having established how animals get their basic components (to some extent) and talked about foodchains, Cox now moved back to DNA and how come there are so many different sorts of organisms. First he gave a brief description of how DNA codes for proteins (with not much detail) and then we talked about what drives mutations. He name checked the sorts of causes, and showed us one – cosmic rays. That was a pretty neat experiment, I don’t know that I’d seen a cloud chamber before and it was cool to see the cosmic rays passing through the vapour in the tank! He then talked about the incredibly high number of combinations of possible DNA molecules that there are if everything was down to random chance – most of which would be instructions for organisms that couldn’t live. So there must be something that constrains the set of combinations, and that something is natural selection.
I found his explanations here to be rather muddy to be honest, perhaps because I would’ve approached the subject differently if I was doing the explanation, perhaps because it was a high level overview of something biological told by a physicist so something got lost in the translation. But we got neat footage of lemurs in Madagascar, so that made up for it for me (and I hope that other people watching it who didn’t know what he was talking about in advance found it comprehensible). The gist of it was right, anyway – that variation between organisms affects their chances of survival (like having a slightly longer thinner finger for an aye-aye makes it easier for it to dig out insects from trees so that makes it easier for it to get food and to stay alive). If something survives more, it has more offspring so there are more like it in the population. And over time these changes can build up (the middle finger of an aye-aye looks really very different to that of other lemurs), and if the population is isolated in some way from the rest of its species then they will become a different species and no longer able to interbreed with the originals. Isolation can be geographical (he showed us how the break up of the supercontinent Gondwana had left Madagascar isolated for tens of millions of years), but it can also be within a geographical area by lifestyle or habitat. (After complaining about his muddy explanations, I think mine probably are too, ah well.)
The fourth episode was all about size, and how the laws of physics affect the size of organisms and the size of organisms affects which laws of physics are important to the organism’s everyday life. He started by swimming with great white sharks (he was in a cage so quite safe, but frankly I would really rather not have that experience personally), and using them to illustrate how the effort required to move through water constrains the shape an animal is – sharks, as with fish and aquatic mammals, are streamlined. He also talked about how living in water allows animals to grow larger, because the water counteracts some of the effects of gravity.
This moved nicely onto a discussion of how on land as animals get bigger they need bigger skeletons to support themselves, and this constrains the sorts of shapes they can be (big animals are proportionally bulkier) and the ways they can move. He illustrated this with Australian marsupials, and worked in an explanation of how kangaroos’ locomotion is so efficient because their elastic tendons store the kinetic energy that they have when they land, and then use that to spring back up again. But the main point of this sequence was to show us the relative femur (thigh bone) sizes of various marsupials both living & extinct. As the length of the bone increases (so the animal is bigger) then the cross-section increases significantly more (i.e. a five-fold increase in length but a forty-fold increase in cross-sectional area) – this is because the mass of the animal has increased in proportion with its volume, and volume increases as the cube of length.
Cox then turned from animals our sort of size (i.e. mice to elephants …) where gravity is the dominant force, and moved to the much smaller scale of insects. Particularly amusing in this bit was him dropping a grape then a watermelon off a balcony to demonstrate that small things bounce and bigger things … don’t. He talked about how this is due both to smaller things falling a bit more slowly (due to friction with the air) and also because big things have more kinetic energy that must be released when they hit the ground (because it’s proportional to mass, I think). And this is done via exploding in the case of the watermelon. So gravity isn’t the big thing for an insect, instead it’s the electromagnetic force, which controls the interactions between molecules – like the way you can pick up a small piece of paper by wetting your finger so the paper sticks to it. This principle is what lets insects walk up walls or across ceilings.
He then went on to talk about what the smallest possible size for an organism is. First for animals – of which the smallest known is a wasp that’s about 0.5mm long, and is a parasite that lays its eggs in the eggs of a moth that feeds on & lays eggs on macademia nuts. And then for bacteria (skipping viruses because they’re not really alive) – where the smallest possible size is 2nm (I think) which is constrained by the size of atoms. You can’t be smaller than the volume necessary to fit all your cellular machinery, and those molecules are the size they are because their atoms are the size they are.
And then Cox talked a bit about how size affects metabolism, and how that in turn affects longevity. Smaller things have a higher surface area to volume ratio (because as something gets longer its surface area goes up by the square of the length change but its volume goes up by the cube). And this means they lose more heat than a larger version. And if you’re an endotherm (like people are) and generate your heat inside you, then the more you lose the more energy you must use to replace it. So smaller animals tend to have a higher metabolism and generate more energy from more food more quickly. Bigger animals both don’t need so much energy (if they’re endotherms) but also there are other constraints that mean that they need to slow down their metabolism. I think one of those was that it takes longer for things like nutrients to get through the circulatory system and so cells at the periphery can’t run too fast otherwise they’d burn up all their resources before they could be replenished (I’m not sure I’ve remembered that right though). Then Cox finished up by using crabs to illustrate that things with a slower metabolism tend to live longer (and this segment made J shudder because he hates crabs!).
The second episode of Brazil with Michael Palin was called “Into Amazonia” and covered (roughly speaking) the north west of the country, including the capital (Brasilia) and some of the indigenous people. The programme was bookended by the two tribes he visited – starting with the Yanomami who are very isolated and trying to remain so and ending with the Wauja who are assimilating some bits of modern Western culture while still preserving their own culture. The leaders of both peoples are worried about the impact that government projects (such as dams and mines) will have on their way of life, and frustrated about the lack of consultation.
Palin also visited one of the last remaining rubber tappers – rubber was a major export from Brazil before the British got hold of some seeds and grew rubber trees in Malaysia. A bit of a sad segment, because the industry has just dried up & gone away. As a counterpoint I think this was where he got to swim with the pink river dolphins, which right up till they showed up I had assumed were going to be some sort of euphemism (particularly with the solemn young man explaining how sometimes girls turn up pregnant & they say the dolphins did it)!
I’m not going to run through everywhere he went or everything he saw, but the other bit that stuck in my mind was Fordlandia. This was a planned town, with a Ford factory, and it was supposed to be a perfect America (this is back in the 1920s). But what it was was a perfect failure, and all the remains today are some abandoned ruined buildings in the jungle.