Life on Earth

Earth formed around 4.54 billion years ago, approximately one-third the age of the universe, by accretion from the solar nebula. Volcanic outgassing probably created the primordial atmosphere and then the ocean, but the early atmosphere contained almost no oxygen. The earliest life forms we know of were microscopic organisms (microbes) that left signals of their presence in rocks about 3.7 billion years old. The signals consisted of a type of carbon molecule that is produced by living things. Oxygen is the gas that is required for the life of most creatures. This is present in the Earth's atmosphere and also in water. Oxygen is constantly put into the atmosphere by plants and trees. Earth's atmosphere also contains a small amount of carbon dioxide. In March 2017, putative evidence of possibly the oldest forms of life on Earth was reported in the form of fossilized microorganisms discovered in hydrothermal vent precipitates in the Nuvvuagittuq Belt of Quebec, Canada, that may have lived as early as 4.28 billion years ago, not long after the oceans formed. Over time, the Earth has changed tremendously, as have the living organisms on our planet. We do not know if life arose once, more than once, or in disparate locations. What we do know, however, is that if we reconstruct the evolutionary tree of every extant organism found on Earth today, they all share the same ancestor.

Life on Earth is mostly green

Life on Earth

How did life begin? There can hardly be a bigger question. For much of human history, almost everyone believed some version of "the gods did it". Any other explanation was inconceivable. That is no longer true. Over the last century, a few scientists have tried to figure out how the first life might have sprung up. They have even tried to recreate this Genesis moment in their labs: to create brand-new life from scratch. So far nobody has managed it, but we have come a long way. Today, many of the scientists studying the origin of life are confident that they are on the right track –they have the experiments to back up their confidence.

There is no question more intriguing than- How life started on Earth

Everything we know about life comes from a sample size of one: life here on Earth. And the fact is, we don’t really know exactly how life arose from inorganic matter all those billions of years ago … and who’s to say that different processes might not have taken place on different planets out there in the universe? Perhaps there are more questions to ask, more possibilities to consider. But let’s start with what we know about some of the very first living things on Earth. Cyanobacteria, also known as blue-green algae, started out on Earth quite a while ago. Possible fossil examples have been found in rocks that are around 3500 million years old, in Western Australia.  

Although commonly referred to as blue-green algae, cyanobacteria are not actually algae. Cyanobacteria, and bacteria in general, are prokaryotic life forms. This basically means that their cells don’t have organelles (tiny structures inside cells that carry out specific functions) and do not have distinct nuclei—their genetic material mixes in with the rest of the cell. This characteristic is distinctive of bacteria and archaea; all other life forms on Earth, including real algae, consist of eukaryotic cells with organelles and with genetic material contained in one place (the nucleus).

Unicellular organisms are believed to be the first life-forms on Earth

At some point on our planet, in the very early stages, the molecules that are abundant and precursors to life, under the right energy and chemical conditions, began to simultaneously metabolize energy, respond to the environment, grow, adapt, evolve, and reproduce. Even if it would be unrecognizable to us today, that marks the origin of life. In a radically unbroken string of biological success, our planet has been a living world ever since.

 

Evolution of Life

 When cyanobacteria evolved at least 2.4 billion years ago, they set the stage for a remarkable transformation. They became Earth’s first photo-synthesizers, making food using water and the Sun’s energy, and releasing oxygen as a result. This catalyzed a sudden, dramatic rise in oxygen, making the environment less hospitable for other microbes that could not tolerate oxygen. After the initial pulse of oxygen, it stabilized at lower levels where it would remain for a couple billion years more. In fact, as cyanobacteria died and drifted down through the water, the decomposition of their bodies probably reduced oxygen levels. So, the ocean was still not a suitable environment for most lifeforms that need ample oxygen.

While microbes can process lots of chemicals, they did not have the specialized cells that are needed for complex bodies. Animal bodies have various cells –skin, blood, bone – which contain organelles, each doing a distinct job. Microbes are just single cells with no organelles and no nuclei to package their DNA.

Cells also began living together, probably because certain benefits could be obtained. Groups of cells might be able to feed more efficiently or gain protection from simply being bigger. Living collectively, cells began to support the needs of the group by each cell doing a specific job.

Hostile terrestrial environment prompted the first life to begin in the water where conditions were more suitable

Dates given are estimates based on scientific evidence.

In biology, evolution is the process by which populations of organisms acquire and pass on novel traits from generation to generation.

Its occurrence over large stretches of time explains the origin of new species and ultimately the vast diversity of the biological world.

Contemporary species are related to each other through common descent, products of evolution, and speciation over billions of years.

The basic timeline is a 4600 million-year-old Earth with 4000 my (million years) of simple cells; 3000 my of photosynthesis; 2000 my of complex cells; 1000 my of multicellular life; 600 my of simple animals; 500 my of fish and proto-amphibians; 475 my of land plants; 400 my of insects and seeds; 360 my of amphibians; 300 my of reptiles; 200 my of mammals; 150 my of birds; 100 my of flowers; and 65 my since the non-avian dinosaurs died out.

Where does evolution stand now

We measure the speed of gene evolution by comparing human DNA with that of other species, which also allows us to determine which genes are fast-evolving in humans alone. One fast-evolving gene is the human accelerated region 1(HAR1), which is needed during brain development. A random section of human DNA is on average more than 98% identical to the chimp comparator, but HAR1 is so fast-evolving that it's only around 85% similar. Realizing evolution doesn't only happen by natural selection makes it clear the process isn't likely to ever stop. Freeing our genomes from the pressures of natural selection only opens them up to other evolutionary processes – making it even harder to predict what future humans will be like. However, it's quite possible that with modern medicine's protections, there will be more genetic problems in store for future generations.

That's it for now. Stay safe. Stay Healthy.