This quote from Edward Robert Harrison – “hydrogen, given enough time eventually turns into humans”. How does it happen?
In: Biology
After the Big Bang, the universe was full of mostly hydrogen gas. Gravity caused this some of this gas to condense into the first stars, and under the intense pressure of gravity, the hydrogen underwent nuclear fusion, changing some of it into heavier elements like helium, carbon, oxygen, silicon, and iron. These early stars died and dispersed these heavier elements throughout the gas clouds that hadn’t yet collapsed into stars.
About 5 billion years ago, one of these gas clouds started to collapse under its own gravity, possibly triggered by the shockwave from a nearby supernova. Most of the gas, which was still mostly hydrogen and helium, condensed into into a star, which we call the sun, while the rest of it formed a swirling disc around the star. Near the sun, it was too hot for light compounds like water or methane to condense, but heavier compounds like silicates and iron, with their high melting points, started to condense into the inner planets of Mercury, Venus, the Earth, and Mars. Further from the sun, it was cold enough for the lighter compounds to form the gas giants, and a variety of icy meteors and asteroids. These icy meteors were sent into the inner Solar System by the gravity of the gas giants, where they crashed into the inner planets, delivering their payloads of water into the atmospheres of each planet. Mercury and Venus were too hot for it to stick around, and Mars’ gravity was too weak to hang on to it, but the Earth retained enough water to have sizeable oceans.
In these oceans, about 4 billion years ago, life emerged. The process is not especially well understood, but early microorganisms were probably little clusters of RNA encased in lipid membranes, that survived by feeding on a chemical soup of sulphide molecules coming from undersea volcanic vents. At some point, some organisms evolved to make use of the abundant carbon dioxide in the atmosphere and the light of the sun, developing photosynthesis. These organisms slowly converted the carbon dioxide rich atmosphere to one that contained oxygen. At around the same time, some of these early single-celled microorganisms started to cluster together in groups, becoming dependent on each other, and eventually evolving into the first multicellular life forms.
These multicellular life forms were quite simple until about 500 million years ago, when there was a massive growth in the diversity of life. Practically every phylum of animals we see today originated during this, the Cambrian explosion, including the very first fish. These new animals diversified and spread out until they occupied every kind of niche the ocean offers, and then some began the movement onto land. The land at this point had plants on it, but the lack of other animals meant there was minimal competition and predation, and the abundance of plants meant there were plenty of niches to exploit. Early land-dwelling vertebrates emerged between about 400 and 350 million years ago.
With land-dwelling animals now commonplace, they spread out and evolved to fit the huge number of available niches. We can skip over most of the history of terrestrial life, suffice only to say that it was dominated by reptiles until about 65 million years ago, when the Earth was struck by a colossal meteor in Chicxulub, Mexico, which wiped out most of the reptiles, leaving only the squamates (lizards and snakes), birds, crocodiles, turtles, and the tuatara. This made room for mammals to take over, with their warm-bloodedness, fur, and ability to birth live young and produce milk.
The mammals evolved to fill the niches left vacant by the extinction of the reptiles. A particular group of mammals, called primates, evolved about 55 million years ago, adapted to living in the trees of tropical forests. This lead to adaptations like visual acuity and large brains (vital for surviving in the complex and dynamic environment of jungle canopy), dextrous grasping hands, and a high range of movement in the limbs. Over time, as these primates spread and the environment changed over time, some of them moved into new environments, that were less densely forested and more open. This encouraged the development of bipedalism, which freed up the hands, which no longer needed to be used for swinging between branches, for picking and carrying food, and which was more efficient for long-distance running. These were the first hominids, and they date back about 7 million years.
Over time, these early hominids started to develop greater intelligence, as the use of tools and their social structures provided an evolutionary advantage. Homo erectus evolved about two million years ago, and is reckoned to be the first hominid capable of using fire. Slowly, over time, this process continued, with hominids becoming more adapted for intelligence, tool use, and long-distance running. This led to the dexterity of our hands and fingers, large heads, and hairless bodies that can sweat.
Anatomically modern humans evolved about 300000 years ago, in sub-Saharan Africa.
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