Question

Describe the possible events in chemical evolution starting with the origin of our planet and leading up to the origin of living systems. Describe how the environment on Earth evolved from its beginnings and through time (you may consider describing the atmosphere and the landscape of the early Earth). Be sure to include the evidence supporting the possible sequence of key events. Include the relative geological time frame for these events.

Answer 1

Earth was formed 4.6 billion years ago from the same nebula cloud of gas and dust that formed the Sun and other planets. Earth back then was very different from Earth now, and it would have been impossible for life to exist on it. The Earth is still changing even today. It has a molten layer, which causes volcanoes to occasionally erupt, and the crust of the planet is constantly moving, sliding over, under, and sideways against itself.

With erupting volcanoes and a constantly moving crust, the Earth is changing even today.

This is our solar system, in one of the arms of the Milky Way galaxy. When the universe began, around 10 billion years ago, Earth wasn't around. Neither was our solar system. Our solar system, the Milky Way, was formed in a perfectly ordinary place in the universe in the normal way. Solar systems and the planets within them form from the spinning disks of matter. Slowly, the grains of matter come together to form clumps, then boulders, and eventually balls big enough to have their own gravity coalesce. At this point, these clumped matter are called planetesimals, which just means a small, irregular-shaped body formed by colliding matter.

Eventually, the planetesimals grew larger by colliding and combining with other bodies of matter. As the planetesimals grew larger, their gravity was greater, and they collected even more matter. Some of the planetesimals began to orbit the main star, our Sun. When they do this, they are considered to be a planet, an astronomical object that orbits a star and does not shine with its own light. Earth formed this way about 4.6 billion years ago and was mostly done in about 10-20 million years, although it still continues to change to this day.

In the 1920s, Russian scientist Aleks and Oparin and English scientist J. B. S. Haldane both separately proposed what's now called the Oparin-Haldane hypothesis: that life on Earth could have arisen step-by-step from non-living matter through a process of “gradual chemical evolution.” ^33start superscript, 3, end superscript

Oparin and Haldane thought that the early Earth had a reducing atmosphere, meaning an oxygen-poor atmosphere in which molecules tend to donate electrons. Under these conditions, they suggested that:

•             Simple inorganic molecules could have reacted (with energy from lightning or the sun) to form building blocks like amino acids and nucleotides, which could have accumulated in the oceans, making a "primordial soup."

•             The building blocks could have combined in further reactions, forming larger, more complex molecules (polymers) like proteins and nucleic acids, perhaps in pools at the water's edge.

•             The polymers could have assembled into units or structures that were capable of sustaining and replicating themselves. Oparin thought these might have been “colonies” of proteins clustered together to carry out metabolism, while Haldane suggested that macromolecules became enclosed in membranes to make cell-like structures.

•             In 1953, Stanley Miller and Harold Urey did an experiment to test Oparin and Haldane’s ideas. They found that organic molecules could be spontaneously produced under reducing conditions thought to resemble those of early Earth.

•             Miller and Urey built a closed system containing a heated pool of water and a mixture of gases that were thought to be abundant in the atmosphere of early earth. To simulate the lightning that might have provided energy for chemical reactions in Earth’s early atmosphere, Miller and Urey sent sparks of electricity through their experimental system.

•             About 2000 million years ago (mya) the first cellular forms of life appeared on earth.

•             The mechanism of how non-cellular aggregates of giant macromolecules could evolve into cells with membranous envelop is not known.

•             Some of these cells had the ability to release O2. The reaction could have been similar to the light reaction in photosynthesis where water is split with the help of solar energy captured and channelized by appropriate light harvesting pigments.

•             Slowly single-celled organisms became multi-cellular life forms. By the time of 500 mya, invertebrates were formed and active. Jawless fish probably evolved around 350 mya. Sea weedsand few plants existed probably around 320 mya.

•             We are told that the first organisms that invaded land were plants. They were widespread on land when animals invaded land.

•             Fish with stout and strong fins could move on land and go back to water. This was about 350 mya. These animals called lobefins evolved into the first amphibians that lived on both land and water. These were ancestors of modern day frogs and salamanders.

•             The amphibians evolved into reptiles. They lay thick-shelled eggs which do not dry up in sun unlike those of amphibians. Again we only see their modern day descendents, the turtles, tortoises and crocodiles.

•             In the next 200 million years or so, reptiles of different shapes and sizes dominated on earth. Giant ferns (pteridophytes) were present along with reptiles but they all fell to form coal deposits slowly.

•             Some of these land reptiles went back into water to evolve into fish like reptiles probably 200 mya (e.g. Ichthyosaurs).

•             The land reptiles were, of course, the dinosaurs. The biggest of them were Tyrannosaurus and Ultrasaurus.

•             About 65 mya, the dinosaurs suddenly disappeared from the earth. We do not know the true reason. Some say climatic changes killed them. Some say most of them evolved into birds. The truth may live in between. Small sized reptiles of that era still exist today.

•             The first mammals were like shrews. Their fossils are small sized. Mammals were viviparous and protected their unborn young inside the mother’s body.

•             Mammals were more intelligent in sensing and avoiding danger at least. When reptiles came down mammals took over this earth.

•             There were in South America mammals resembling horse, hippopotamus, bear, rabbit, etc. Due to continental drift, when South America joined North America, these animals were overridden by North American fauna. Due to the same continental drift pouched mammals of Australia survived because of lack of competition from any other mammal.

•             About 15 mya, primates called Dryopithecus and Ramapithecus were existing. They were hairy and walked like gorillas and chimpanzees. Ramapithecus was more man-like while Dryopithecus was more ape-like.

•             Few fossils of man-like bones have been discovered in Ethiopia and Tanzania. These revealed hominid features leading to the belief that about 3-4 mya, man-like primates walked in eastern Africa. They were probably not taller than 4 feet but walked up right.

•             Two mya, Australopithecines probably lived in East African grasslands. Evidence shows they hunted with stone weapons but essentially ate fruit.

•             Some of the bones among the bones discovered were different. This creature was called the first human-like being the hominid and was called Homo habilis. The brain capacities were between 650-800cc. They probably did not eat meat.

•             Fossils discovered in Java in 1891 revealed the next stage, i.e., Homo erectus about 1.5 mya. Homo erectus had a large brain around 900cc. Homo erectus probably ate meat.

•             The Neanderthal man with a brain size of 1400cc lived in near east and central Asia between 1,00,000-40,000 years back. They used hides to protect their body and buried their dead.

•             Homo sapiens arose in Africa and moved across continents and developed into distinct races. During ice age between 75,000-10,000 years ago modern Homo sapiens arose.

•             Pre-historic cave art developed about 18,000 years ago. Agriculture came around 10,000 years back and human settlements started. The rest of what happened is part of human history of growth and decline of civilisations.