Life on earth appeared at least 3.5 billion years ago, but early life experienced a long life in the ocean and was in a boring single-cell state. It was not until the Cambrian period (541 million to 485 million years ago) that competition among living things became more and more obvious, and life began to find ways to "equip" itself to adapt to the environment and win in the competition.
During this period, biological characteristics continued to evolve like mushrooms after a rain. People call this event the Cambrian Explosion of Species. All the major animal categories we see today appeared at that time, including the protagonist of this article - arthropods.
However, the stage of Cambrian life was in the ocean.The terrestrial environment is no less suitable for life than the moon, is completely inconsistent with the survival and reproduction of life.
If life wants to enter the land from the ocean, it must make several preparations:
First, they need a movement system that can support them going back and forth, just like a return capsule is needed to land on the moon. A one-way ticket is meaningless; second, a protection system that can withstand the pressure of the terrestrial environment, just like astronauts need space suits; third, they also need a special life support system to support their lives in places full of radiation, with huge temperature differences, and even without the oxygen they are familiar with.
at last,Even if creatures are ready, they still need a reason, they adapt to the ocean, the ocean is comfortable for them, and they need a reason to risk their lives and enter the deadly environment.
Many people may be curious about which animal first entered land? This is actually an uncontroversial part of the biological world.Arthropods were the first animal groups to land, because we were ready to log in at the earliest.
They had a locomotor system suitable for traveling back and forth very early. Their body structure is simple, including a slightly complex head and many segments, each with a pair ofappendages, these appendages allow them to crawl and dig in rocks and mud on the seabed, and of course are also suitable for traveling between land and sea.
During the Cambrian period, they acquired a set of protective equipment——exoskeleton, this structure protects them from predators swimming above, but also gives them a "space suit" to land.
The Earth has a disproportionate natural satellite, the Moon. Under its gravitational influence, the oceans experience periodic ebbs and flows, which play an important role in the evolution of life on Earth.
Those arthropods that live along the coast and in shallow water are forced to get a chance to come ashore when the tide recedes.
Now, they only have one more reason to move deeper into the land.Nowadays, it is generally believed that this reason is to avoid predators.,In order not to be eaten, they must bravely take steps towards land.
However, they still lack the last preparation, which is a system that can be used for terrestrial breathing, and it is possible to move towards the new world.
One of the earliest known terrestrial animal fossils is an arachnid arthropod from the Late Silurian Period (about 420 million years ago) - the scorpionHowever, they cannot be fully called "terrestrial animals" because they lack the respiratory system to breathe oxygen in the air on land.
They have what are called "book gills” respiratory organ, this respiratory organ has many flat respiratory plates that are stacked like the pages of a book, hence its name.
Since it is named after "gills", this already implies that it is an organ that obtains oxygen from the water, but these scorpions can obviously move and forage outside the water along the coastline.Because the tarsi at the end of their steps are shorter and thicker,There is no buoyancy provided by sea water on land, and the body needs to be supported entirely by the legs. This kind of walking means that they are more suitable for crawling on land than their counterparts.
As for how the book gills support them out of the water?
Perhaps this structure just needs to be kept moist to get good oxygen, so as long as these scorpions don't travel too far, it's fine.
It was not until the Devonian period (417 million to 358 million years ago) that arachnids evolved from book gills.book lung” and are fully adapted to terrestrial activities.
Almost at the same time as arachnids moved to land, there was another branch of arthropods, that isMyriapoda,Including today's millipedes and centipedes, which landed almost at the same time as scorpions.
But their "equipment route" is different from that of arachnids. They do not have book lungs, but have acquiredtracheal respiratory system, delivering oxygen directly to the organs through the stomata.
Compared with arachnids, little is known about the landing evolution of myriapods, but their ancestors will also evolve into a land animal in the near future. They can be called the most successful group in the history of the earth, namely insects.
at last
In many popular science books, when depicting the landing of life on Earth, they usually subconsciously believe that plants entered land earlier. However, judging from the existing fossil evidence,Arthropods may have entered land earlier than plants.
The earliest arthropods that came to land were not herbivorous, but were predators. For example, all scorpions and centipedes are predators to this day. They originally lived on land without plants.
There is an important reason that prevents plants from landing animals, that is, animals can survive in harsher environments, but plants cannot.
For example, the ozone layer gradually became what it is today with the landing of plants. Before the landing of plants, the concentration level of the ozone layer was lower, which means that it absorbs less ultraviolet radiation from the sun, which is very unfriendly to life. It is difficult for animals and plants to be exposed to sunlight when the concentration level of the ozone layer is low.
However, animals can change their lifestyles to adapt to this situation, such as hiding during the day and being active at night, but plants cannot. They must need sunlight to survive.
Apparently this is what scorpions and centipedes do, and they are nocturnal to this day, while plants can only wait until ozone levels are high enough.