We still have little idea how, when, or where life began. The evidence is circumstantial and can be compared with delving into such records as there are in Massachusetts of the Mayflower, to discern the origins of the life.
- 5,000 years ago The Bible states God created humans & higher organisms.
- In the mid-1800s Creationism & insects, frogs & other small creatures arise spontaneously from mud & rot.
- Pasteur demonstrated bacteria & other microorganisms arise from parents resembling themselves. Spontaneous generation is dead.
- Darwin proposed natural selection, the theory that environmental pressure results in the perpetuation of certain adaptations. The evolution of complex organisms therefore possible, & all current life forms could have evolved from a single (last) common ancestor.
- Darwin (privately) suggested life could have arisen from chemistry: “in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present.
- 1953 Miller-Urey experiment (U. Chicago) demonstrates that amino acids could be formed with atmospheric gases + lightning.
- In the late 1960s Woese (U. Illinois), Crick (England), Orgel (Salk Inst, San Diego) concurrently proposed RNA may have preceded proteins & catalyzed all reactions for survival & replication of ‘last common ancestor’. The ‘RNA World’ hypothesis was born.
- In 1977 Hydrothermal vents on the seafloor discovered teaming with diverse life. This suggests the possibility life may not have evolved at the surface.
- In 1983Thomas Cech (U. Colorado) & Sidney Altman (Yale) independently discovered ribozymes, enzymes made of RNA. Heritability & reproducibility is possible with a single molecule.
- In 1988 Gunter Wachtershauser (German patent lawyer!) theories that Fe & Ni sulfide minerals at hydrothermal vent systems provided the template & catalyst for the formation of biological molecules.
- In 1997 Jay Brandes (Carnegie Inst.) demonstrates that N2 is converted to NH3 in the presence of H2 & magnetite (Fe3O4), at T & P typical of hydrothermal vents. Mineral surfaces and HT vent environments can produce a biologically useful form of N.
- In 2000 Cody et al. demonstrate the synthesis of pyruvate using mineral catalysis under hydrothermal conditions. Pyruvate is a branch point for many extant biosynthetic pathways.
Theories on the origin of life
Life on Earth began more than 3 billion years ago, evolving from the most basic of microbes into a dazzling array of complexity over time. But how did the first organisms on the only known home to life in the universe develop from the primordial soup?
One theory involved a “shocking” start. Another idea is utterly chilling. And one theory is out of this world! Inside you’ll learn just how mysterious this all is, as we reveal the different scientific theories on the origins of life on Earth.
Perhaps life did not begin on Earth at all but was brought here from elsewhere in space, a notion is known as panspermia. For instance, rocks regularly get blasted off Mars by cosmic impacts, and a number of Martian meteorites have been found on Earth that some researchers have controversially suggested brought microbes over here, potentially making us all Martians originally.
Other scientists have even suggested that life might have hitchhiked on comets from other star systems.
However, even if this concept were true, the question of how life began on Earth would then only change to how life began elsewhere in space. Oh, and if you thought all that was mysterious, consider this: Scientists admit they don’t even have a good definition of life!
Instead of developing from complex molecules such as RNA, life might have begun with smaller molecules interacting with each other in cycles of reactions.
These might have been contained in simple capsules akin to cell membranes, and over time more complex molecules that performed these reactions better than the smaller ones could have evolved, scenarios dubbed “metabolism- first” models, as opposed to the “gene-first” model of the “RNA world” hypothesis.
Nowadays DNA needs proteins to form, and proteins require DNA to form, so how could these have formed without each other? The answer may be RNA, which can store information like DNA, serve as an enzyme like proteins, and help create both DNA and proteins. Later DNA and proteins succeeded in this “RNA world,” because they are more efficient.
RNA still exists and performs several functions in organisms, including acting as an on-off switch for some genes. The question remains how RNA got here in the first place. And while some scientists think the molecule could have spontaneously arisen on Earth, others say that was very unlikely to have happened.
Other nucleic acids other than RNA have been suggested as well, such as the more esoteric PNA or TNA. A study in 2015 suggests the missing link in this RNA puzzle may have been found.
Ice might have covered the oceans 3 billion years ago, as the sun was about a third less luminous than it is now, scientists say.
This layer of ice, possibly hundreds of feet thick, might have protected fragile organic compounds in the water below from ultraviolet light and destruction from cosmic impacts. The cold might have also helped these molecules to survive longer, allowing key reactions to happen.
The deep-sea vent theory suggests that life may have begun at submarine hydrothermal vents spewing key hydrogen-rich molecules.
Their rocky nooks could then have concentrated these molecules together and provided mineral catalysts for critical reactions. Even now, these vents, rich in chemical and thermal energy, sustain vibrant ecosystems.
The first molecules of life might have met on clay, according to an idea elaborated by organic chemist Alexander Graham Cairns-Smith at the University of Glasgow in Scotland.
These surfaces might not only have concentrated these organic compounds together but also helped organize them into patterns much like our genes do now.
The main role of DNA is to store information on how other molecules should be arranged. Genetic sequences in DNA are essentially instructions on how amino acids should be arranged in proteins.
Cairns-Smith suggests that mineral crystals in clay could have arranged organic molecules into organized patterns. After a while, organic molecules took over this job and organized themselves.
Lightning may have provided the spark needed for life to begin. Electric sparks can generate amino acids and sugars from an atmosphere loaded with water, methane, ammonia, and hydrogen, as was shown in the famous Miller-Urey experiment reported in 1953, suggesting that lightning might have helped create the key building blocks of life on Earth in its early days.
Over millions of years, volcanic clouds in the early atmosphere might have held methane, ammonia, and hydrogen and been filled with lightning as well.