David Attenborough's Life on Earth is a landmark milestone in nature documentaries. The book counterpart is filled with incredible stories and weaves an engaging narrative through evolution's history. It also is filled with fun facts about nature that make you want to turn to the nearest person and tell them about it: "Hey, did you know?".
After a while of doing this I decided to start writing them down. I'm sure he wouldn't mind.
The Infinite Variety
- Sponges are just clumps of cells that choose to grow together. Force one through a fine sieve to break it down back into individual cells, and it will put itself back together into a new sponge. Take two sponges and mix them and they'll turn into one.
- The fertilised egg of a jellyfish does not immediately turn into a small jellyfish, but first grows into a plant / flower-like organism called a polyp. This looks uncannily like a small tree that has branches where each flower "ripens" into mini jellyfish that detach and swim away.
- The Portugese man o' war (which I have seen in person and is quite beautiful) is not a variation of the "medusae" swimming form of jellyfish. It isn't even a single organism, but instead a floating colony of solitary polyps from a gas sack.
Building Bodies
- A mollusc may discharge as many as 400 million eggs. This is mostly because it spends its childhood drifting without a shell.
- Squids change colour and shape to communicate. A female squid has been filmed signalling to a male lying alongside her that she is not ready to mate, while at the same time displaying a pattern on the other side of her body to summon another male. The nerve.
- The sea-pig, a specialised sea cumber, may defend itself by extruding their internal organs. Over the next few weeks it will slowly grow itself a new set of entrails.
The First Forests
- Mosses are one of the most primitive plants, being no more than permeable water-filled cells. They achieve "rigidity" by packing together but don't have stems or roots. Mosses alternate between asexual and sexual generation.
- Scorpions are one of the earliest examples of mating rituals. They reproduce by suddenly grabing the female pincers for protection, they shuffle around to clear the ground, dumps a bunch of sperm on the ground, then heaves the female above the sperm packet, then go their separate ways.
- Spiders do all sorts of things. Some spin a triangle of silk, attach their sperm to that, holds it in a special limb, and does everything from twanging the web of a female with a secret handshake to offering gift-wrapped bugs (and will tie her to the ground whilst she's distracted). If he's lucky he'll find her opening, deposit his sperm and run away before he gets eaten.
- Dragonflies, one of the most ancient examples of winged insects, can only rotate their wings up and down and cannot fold them back. They use hair at the front of their body to sense if they're flying straight. Because the wings can't really rotate, the hum you hear is the strain of them making sharp turns.
- The pine tree approach of hoping that wind blows pollen into a pine cone is ridiculously inefficient, so it produces millions of grains that you can tap and produce a golden cloud, and cover entire ponds with curds of it - all wasted.
The Swarming Hordes
- The job of a grub, maggot, or caterpillar is simply to eat. Its body is dedicated to this, with no sexual equipment, no communication (send or receive sight, smell or sound), and barely any legs to serve its swelling bag of food.
- When a larva develops from an egg, one group of cells builds the body of the caterpillar, the other stops dividing and remains generic in dense clusters. Its body doesn't grow, but just swells with food. Finally, the dormant cells begin to divide to build a new body. E.g. the maggot's future adult organs (the fly) exist merely as patches of skin within the maggot's body. The brain and central nervous system is preserved, and the adult may remember things from its larvae form.
- As the adult emerges, it convulses and pumps blood into the veins of its baggy wings, causing them to expand and the patterns to come clear. If you damage the tip of a wing at this poing, it will drip blood. Gradually the blood is drawn back in and the veins harden into dry, rigid struts to give the wing strength.
- The adult no longer build their bodies and most do not, or rarely feed (e.g. nectar for energy). Now it is time to mate.
- Our ears cannot hear breaks between sounds of less than 1/10th of a second. Cicadas are able to distinguish sound intervals of 1/100th of a second. Their song varies frequencies from 200 to 500 per second in a rhythmic way. Mosquitos use a similar mating call as fast as 1/500th of a second which is the hum you hear when you're trying to sleep.
- Most insects are limited by size, due to their tracheae (which works through gaseous diffusion) becoming less efficient as the length of the tube increases.
- Some slave-making ants have such large jaws that they cannot feed themselves and are dependent on their slaves.
The Conquest of the Waters
The Big Picture
The book goes through the varied branches of the big tree of evolution, finally ending up with humans. It starts with DNA and its ability to self-replicate, moves up to cells, sponges, wormy things, fish, amphibians, reptiles, mammals, and finally humans. There's quite a bit he covers in-between that didn't get the Attenborough treatment at my highschool, so I've decided to write my own summary.
The order shown here is roughly the same as the book, and represents branches of evolution - i.e. the roads not taken if you goal is to end up as a human.
- Self replicating DNA. Mutations accumulate at a constant rate, so it can be used as a clock to estimate the branching of species.
- Chemoautotrophs / bacteria that take carbon compounds from the primordial seas
- Photosynthesising bacteria with flamboyant colours that figured out how to create their own energy from the sun using hydrogen (from volcanic eruptions), still chilling out in the hot springs in volcanic areas.
- Blue-green cyanobacteria that started using hydrogen from water instead, creating oxygen as a byproduct. Still around in Shark Bay, Australia. This created our ozone layer to protect from UV rays.
- Protozoa. A single cell with mitochondria that burns oxygen and / or chloroplasts. Half animal, half plant. Cool. Because mitochonria / chloroplasts are themselves independent organisms (with their own DNA), a protozoa is a "committee" of simpler organisms. Also invented thousands of shapes and features including the flagellum (i.e. swimming tail), cilia, and silica shells. When splitting for reproduction, it creates one large comparatively immobile cell (egg), and a smaller flagellum powered one (sperm). These unite again to mix genes and accelerated evolution.
- Sponges. Instead of a committee of organisms inside a single cell, they've figured out how to have coordinated colonies of cells.
- Jellyfish. Invented contracting cells (muscles) and trasmitting electric impulses (nervous system). Jellyfish have two forms in their lifecycle, the static plant-like polyp form, and the swimming medusae form. Soft coral, hard corals (typically only a few mm across that poke their heads out of their limestone cells), and anemones are examples of polyps without a medusae stage.
- We fork into 3 main invertebrate branches here: all of which invented bilateral symmetry (and left and right side of the body). The first group grows up to be radially symmetrical: like sea lilies, starfish and sea urchins. It contains the echinoderms - a central limestone body / stem called a calyx with five branching arms. They also figured out mouth / anus combos.
- The second are shelled animals like clams, cowries, and sea snails. They start with flatworms which breathe through the skin, move with cilia or undulations, and with light sensitive spots (the beginnings of a head). They then invented shells (brachiopods, molluscs, which recently have ended up as snails and slugs and in our gardens), then some uninvented shells (sea slugs), others figured out gas chambers in shells to float and use jet propulsion with tentacles (nautilus - still around, ammonites - not around), and then gave up their shells and created cephalopods (squids, cuttlefish, octopus).
- The third, which is closer to the flatworms and snails than the echinoderms, (which we branch from) are elongated segmented (probably for burrowing) animals, starting with annelids / segmented worms. The body itself is segmented into compartments with their own organs and appendages. They developed limestone exoskeletons, jointed legs, and insect-like eyes (trilobites, and later horseshoe crabs). Other branches ended up with crustaceans: the crabs, shrimps, prawns, and lobsters. Except for barnacles, who gave up on swimming and reverted back to a static existent. The modified appendages created legs, tweezers, paddles, egg carriers, and so on. The external skeletons allow some of these to store oxygen and venture on land.
- We take a detour to the plant kingdom, covering the most primitive plants like liverworts and mosses. Mosses have also figured out how to use egg and sperm cells, but rely on water for transport (raindrops, stuck to insects, etc). Moss cells are mostly bags of water (with no rigidity to create a "stem"), but eventually some basic land plants figured out thick-walled cells that conduct water.
- Our ancestors, the segmented worms came up on land as huge 2-meter long millipede to graze on primitive plants. Whereas crustaceans have gills, millipedes developed tracheae - breathing tubes from their shell that branch internally into a fine network across their tissues and organs. They also invented sex without water (where sperm could swim) by intertwining their bodies.
- From millipedes follow centipedes, scorpions, and spiders. Mostly reducing / losing signs of their earlier segmentation.
- Club mosses, horsetails, and ferns came about and invented lignin, the basis of wood. They started to grow taller with stems and leaves.
- Amphibians...
- More insects, like bristletails, springtails, and silverfish. Still segmented. They stabilise on six legs and tripartite bodies: usually a head with a mouth and sensory organs, a thorax with muscles for legs / wings, and abdomen for digestion and reproduction, all enclosed in a chitin external skeleton. The external skeleton can be sculpting agnostic of internal anatomy, and so insects develop all sorts of crazy adaptations.
- Insects could probably climb up these taller plants, but possibly due to the difficulties of getting down, this helped develop flight. The living silverfish has two flap-like extensions of its shell that look like rudiments of wings. One theory is that these thin flaps with veins possibly started as a way to quickly warm themselves from the sun, and rotate to face the sun.
- At some point we got dragonflies (carnivorous) and potentially before that cockroaches, grasshoppers, locusts and crickets (herbivorous).
- Cycads and conifers (pines, cedars, firs, etc) come about. They grow even taller and use the wind to reproduce. Now, spores (now called pollen) are blown into cones (which contain eggs).
- Rather than two pairs of wings (dragonflies), we now get bees, wasps, flying ants, and flies. They hook together their wings to produce one pair (butterflies have fused together). Beetles turn one pair into armored protection for the other.
- Magnolias work with beetles (who eat pollen) for reproduction. They accept pollen as soon as their flowers open, but stagger creating their own pollen until later (by which time they are likely already cross-fertilised).
- Plants develop nectar to entice insects to do more fertilisation. Many plants develop bright colours, scents, and sexy shapes to advertise themselves.
- More advanced insects no longer have larvae that resemble the adult (maggots / flies, grubs / beetles, caterpillars / butterflies)
- Swarm insects debut. Some hunting wasps are solitary, and build mud cells to lay eggs, and after storing food (e.g. a delicious collection of paralysed spiders) it abandons it. Some closely related females may build nests close to one another, and sometimes abandon building their own, and join another to build theirs instead. Eventually, one female becomes dominant and lays all the eggs, and others concentrate on cell-building and food. In this way, we see the transition to colonial mentality.