[The Landing Operation of Living Things: Animal Edition] Prologue
I am Takashi Hamachi, a biologist studying evolution. I am working on issues such as the evolution of reproduction and the emergence of multicellular organisms, but I am also interested in other evolutionary problems. In particular, the major event that occurred about 500 million years ago, when living things emerged onto land, is one of them.
I previously wrote about the advancement of plants onto land.
Sometimes, while looking out the window, I wonder what this landscape was like before human activity intensified.
Whether it is the Kanto region where I live, or Kyoto where I spent time until recently, there must have originally been thickets or forests spreading out here and there.
And I think there was an ecosystem within that, with animal food chains and the decomposition by fungi and microorganisms taking place.
Now, 'when' did that ecosystem exist?
I don't mean in the land you are looking at right now.
I mean, when did the 'green' view that you take for granted as the terrestrial landscape you look at under the sky become the norm?
I wrote about how bryophytes clung to the water's edge, lichens waited for rain on dry rock surfaces to prepare the soil, and finally, ferns containing 'rivers flowing toward the sky' called vascular bundles appeared.
After vascular plants took over the green earth, they rapidly fixed carbon dioxide from the atmosphere and piled up giant trees using a material called lignin, which was impossible to decompose at the time, leading to the emergence of Carboniferous forests. Oxygen levels soared, and the Earth's landscape was completely transformed.
Plants went up to land. Then, what about animals?
" Amphibians emerged from fish, and from there, reptiles eventually appeared" is something we learn in school. Textbooks mention things like the appearance of tetrapods, lung breathing, and adaptation to dryness.
There is one question here that we tend to forget.
When they went up to land, what did they eat?
In the water, big fish ate small fish. Small fish ate plankton and even smaller fish. The food chain revolved underwater.
However, the moment they went up to land, that chain was broken.
Was there anything to eat on the land away from the water's edge?
There are plants. A green earth is spreading out.
But eating plants is actually not that simple. Plants are fibrous, difficult to digest, and contain toxins. At least, current plants do. For herbivores to appear, all sorts of 'capital investment' such as modification of the digestive tract, symbiosis with intestinal bacteria, and specialization of oral processing are required.
It is quite unreasonable for early terrestrial vertebrates to suddenly live by eating plants. In fact, most amphibians are carnivorous.
Then, what did they eat?
Yes, 'bugs'.
To be precise, arthropods (insects, spiders, centipedes, and woodlice). The caricature of a frog sticking out its tongue at a fly is impressive.
Arthropods had advanced onto land before vertebrates. It is thought that they ate, decomposed, and multiplied on the organic matter (fallen leaves, pollen, humus) created by plants. And they likely covered the earth with their small bodies, possessing exoskeletons resistant to dryness.
When adult amphibians went up to land, it is thought that insects, which are 'energy in an edible form,' were already walking around there.
Eating plants directly is difficult. However, if you eat insects that have grown by eating plants, that is carnivory. If you are a carnivore, you can use the digestive system you have had since your days as a fish just as it is.
Insects converted the low-calorie, hard-to-digest resource of plants into 'predatable, high-nutrient packets,' scattered them across the terrestrial landscape, and made the land an attractive place for vertebrates.
Let me give you an analogy.
When moving to a new land, what do you need?
Housing, water, and food, of course.
When pioneers step into the wilderness, it is rare for crops to be growing there already.
But what if someone had already started farming, a market had been set up, and a bakery had been built?
Those who arrived later wouldn't need to till the fields themselves; they could just buy bread and eat it.
The advance of vertebrates onto land was exactly that kind of scenario.
Arthropods used plant resources to become the 'bread' for vertebrates. I want you to consider the story that vertebrates came onto land as customers of that shop.
Of course, this is not a story about 'motivation.' The ancestors of amphibians did not think, 'I'll go up on land because there are insects there.' This is a point that often causes confusion, but many factors are involved in a major event like landing. It might have been the ability to move on tidal flats, or perhaps the development of respiratory auxiliary organs was necessary.
However, whether or not they could continue to live in that place after landing—that depends on whether a sustainable source of prey energy exists.
It is thought that arthropods supported that 'sustainability.'
If we organize things this way, the terrestrialization of vertebrates can be re-examined not just as a 'history of the invention of limbs and lungs,' but as a problem of how the 'fuel supply infrastructure' of the ecosystem was established.
Plants create organic matter as primary producers. Arthropods eat them, multiply, and disperse into the space. By preying on those arthropods, vertebrates 'recover' their investment in high-cost physical equipment (limbs, lungs, desiccation resistance).
The 'march of the backboned animals'—fish to amphibians to reptiles—actually reveals a completely different scene when you add the perspective of 'who set the stage.'
Plants created the green earth, arthropods were reinterpreted as packages of those resources, and then vertebrates appeared as 'heavily equipped predators.'
This sequence is a powerful story.
(To be continued)