bio0: How did life first form? (v1.0)

A key reference is Newsweek. 

 A long abiding mystery is how did life first form from the soup of chemicals in primordial earth 3.5 to 4 billion years ago? There is no definitive answer but there is a definite hypothesis. 

Scientists believe the first forms of life on Earth emerged over 3.5 billion years ago from inert geological materials. The exact location of this emergence is still unclear, but many scientists believe that the earliest life forms appeared around deep-sea hydrothermal vents.

Hydrothermal vents are like rocky chimneys on the ocean floor which spew out plumes of superheated fluid from magma beneath the Earth's crust. "Hydrothermal vent sites, and in particular alkaline hydrothermal vents, are unique in bringing together several key threads in origins of life theories together," Jon Telling, a reader in biogeochemistry at Newcastle University in the U.K, says.  "First, they provide a continual and free source of energy to drive the synthesis of relevant organic molecules through the presence of strong chemical gradients. Second, they contain abundant metals such as iron and nickel, that are also abundant in evolutionary ancient proteins that are present in microorganisms today. Finally, attempted reconstructions of the 'Last Universal Common Ancestor' of all life on Earth suggest that it was a thermophile or hyperthermophile ('liked it hot'), used hydrogen gas for energy ('food'), and could harness proton gradients to drive biochemical reactions; all consistent with the mixing of alkaline hydrothermal fluid with more acidic ocean or surface water."

But for life to form at these vents, several key ingredients had to come together. The working hypothesis is the idea that many key components for life could have simultaneously formed early on and combined to make living cells. Researchers have one by one been demonstrating individual components. Broadly, scientists have focused on molecules that either store and transfer biological information (e.g. nucleic acids, DNA or RNA); catalyze reactions within cells (e.g. amino acids - the building blocks of proteins); [or] make cell membranes, giving a cell its own chemical identity, separate from the external environment (e.g. fatty acids). These fatty acids, is the focus of Telling's latest paper, led by postdoctoral research associate at Durham University, Graham Purvis. 

Fatty acids are long organic molecules with a hydrophilic end that attract water and a hydrophobic end that repel water. Therefore, when they are placed in a watery environment they automatically come together in cell-like compartments, with the water-attracting parts on the outside and water-repelling parts on the inside. It is these molecules that would have formed the first cell membranes, isolating and protecting the inner workings of the cell from the world around it. "Our research has shown that fatty acids alongside a range of other different organic molecules can be formed on iron-mineral surfaces, from a stream of pressurized hydrogen gas and dissolved carbon dioxide within a hydrothermal vent fluid," Telling said.

But, had these molecules remained stuck on the surface of the iron-minerals, life on Earth might have never formed. "However, hydrothermal fluids can be dynamic environments, with changing flow paths and proportions of mixing between surface or seawater and the hotter hydrothermal fluid," Telling said. "Crucially, we think that when changes in fluid mixing cause the fluid to become less acidic, some organic molecules such as fatty acids should become electrically negatively charged, as will the mineral surface, effectively repulsing each other. These organic molecules should then lift off the mineral surface into the water in what has been described as an 'electrostatic explosion,' and then (as fatty acids have both a 'water loving' end and 'water hating' end) spontaneously form fatty acid-based membrane-bound spheres, the precursors of 'protocells' (the precursors to biological cells)."

In their study, published in the journal Communications Earth and Environment, the team were able to demonstrate just how easily these long-chain fatty acids can form under the replicated conditions of a hydrothermal vent. "[This study] opens the door to the next phase of experiments: proving that they can 'lift off' and spontaneously form membrane-bound spheres'; a key steppingstone to protocells," Telling said.

But life does not exist without reproduction. For this we need RNA/DNA. A big question is how did RNA/DNA first form? RNA, or ribonucleic acid, is a molecule that is essential to life as we know it. According to the RNA World model (one of many), RNA might have come before DNA and proteins and acted as the ancestral molecule of life. Scientists think RNA building blocks (nucleotides) emerged in a chaotic soup of molecules on early Earth likely in hydrothermal vents deep in the ocean. These nucleotides bonded together to make the first RNAs. No sooner were they made, than they broke down; however, new ones were made in their place. Some RNAs turned out to be more stable than others. These RNA strands grew longer and bonded nucleotides more quickly. Eventually, RNA strands grew faster than they broke down. This was RNA’s chance to start life. In fact, scientists at the Scripps Research institute have synthesized RNA enzymes that can replicate themselves without the help of any proteins or other cellular components and the process proceeds indefinitely.  Molecules that can make a copy of themselves are thought to be crucial to understanding this process as they provide the basis for heritability, a critical characteristic of living systems. How DNA formed is much more unclear, with many theories. 

These discoveries could inform biochemical questions about how life began. However, the origins of life on Earth are still a mystery, and there are many theories about how it might have happened. 

Similar conditions to hydrothermal vents have been proposed to exist on other planets and moons in our solar system: "At the bottom of ice-covered oceans, such as within Jupiter's moon Europa, or Saturn's moon Enceladus, or in the past at active hydrothermal vent sites on Mars, for example," Telling said.

As the first paleontology essays says, the first cells were prokaryotes (examples are bacteria or cyanobacteria), which had no nucleus, mitochondria or most of the other distinct organelles that characterize the eukaryotic nucleated cell (but did have a DNA strand). It took over a billion years after prokaryotes emerged for eukaryotic nucleated cells to first appear!!