How did life first came about? Our planet started out as a molten magma ball too hot for life, but the oldest fossils indicate that it was established soon after Earth had cooled sufficiently to be covered by oceans of water suitable for life. Evolutionary theory allows us to understand how life, once arrived, diversified over deep geological time into the abundant life forms we observe today. However, how life first evolved on Earth remains largely unknown. We have only ever observed life to spring from existing life, and we have yet to find convincing evidence that life exists elsewhere beyond our planet. And although we have extensively modified the DNA of existing organisms, including the synthesis of a simple microbe (C. A. Hutchison III et al., Science 351, aad6253 (2016). DOI: 10.1126/science.aad6253), we have yet to synthesise life from scratch, from its basic organic molecular building blocks such as amino and nucleic acids.
All the diverse life forms on Earth today, including us, appear related to one another and to share a common ancestor. This gives rise to the concept that ‘all life is one’, which stems from the fact that all life is based on DNA and RNA, the double and single-helical molecules that contain the code of life. Similarities in genetic DNA and RNA makeup, as well as the many shared biochemical processes across many different organisms, suggest that we and all other life forms descend from a distant common ancestor. This shared great-great-greatest of grandparents to everything living today is known as life’s Last Universal Common Ancestor, or ‘LUCA’.
Charles Darwin commented on this profound concept that ‘all life is one’ in his book On the Origin of Species, published in 1859:
It is a truly wonderful fact – the wonder of which we are apt to overlook from familiarity – that all animals and all plants throughout all time and space should be related to each other…Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed.
LUCA is Darwin’s ‘primordial form into which life was first breathed’ and from which all life forms on Earth descended. Hence, the myriad species we see today are each from a long line of descent that includes many now-extinct ancestors over the aeons of deep geological time and which converge all the way back to LUCA.
There have been many ideas put forth on where and how LUCA evolved, but the upshot is we do not know. What we do know is that LUCA must have evolved sometime between when Earth first became habitable (4–3.8 billion years ago) and the oldest fossil life forms yet found, in rocks 3.4 billion years old.
The Components of Life
These oldest fossils reveal the overall cell morphologies, but tell us little about the biology of these early organisms. However, they are generally assumed to represent the simplest single-celled microorganisms living today. But even the simplest bacteria today include many complex organic structures (DNA, RNA and ribosomes) and intricate biochemical processes that occur within their cell-wall structures. Our general understanding of evolution suggests that the complex biology represented by these fossil organisms evolved gradually over hundreds of millions of years, from the modification of existing, simpler structures. These first organic structures were not in themselves living entities, but they may have evolved as molecules by way of random mutations and natural selection in much the same way that Darwin proposed species do today. These selective forces may have promoted the merging of different ‘molecular species’ in ways that enhanced their mutual stability and replication in what could be viewed as a continuum leading up to the first bona fide life forms, out of which LUCA would evolve to give rise to all life as we know it.
Earth’s surface had an abundance of all the elements necessary for life: carbon, nitrogen, oxygen, phosphorus, etc. It also included organic compounds, such as amino acids, sugar and sugar alcohols, either formed naturally on Earth or delivered by meteorites. Besides water, life forms are mostly made up of amino acids organised into many different proteins, which are the large, complexly folded, three-dimensional organic compounds that make up our blood, muscles, skin, hair, etc.
Amino acids existed before life, but how did they organise into complex proteins under the direction of DNA and RNA housed within cellular membranes?
One idea is that early organic compounds may have included simple, self-replicating molecules, with those that replicated the fastest or most accurately persisting instead of perishing. Gradually, compounds as complex as small RNA-type molecules may have emerged whose information-storage capabilities could code for different amino acids in the production of proteins essential for a broad spectrum of biochemical functions. For example, phospholipid protein molecules capable of forming impermeable bubbles may have been precursors to cell membranes providing barriers to the outside world. Over time, incremental Darwinian selection led to improvements in replication and the coordinated merging of different organic structures and protein enzymes into the first simple cellular ensembles capable of extracting energy to grow, replicate and evolve, which – in a nutshell – is what life does. Of course, much of the above generalised scenario is highly speculative and we are a long way from understanding the many intricate processes by which life arose on Earth.
Where on Earth might life have first evolved? A likely ‘primordial soup’ from which life was concocted is in the vicinity of volcanic hot springs located along the axis of the mid-ocean ridge mountain chain in the deep, dark ocean. We call these volcanic hot springs ‘black smokers’ because they spew turbulent smoke-like billows of black sulfide particles through mounds and chimney-like columns.
The mid-ocean ridge represents the most recently formed oceanic crust. The large temperature contrast between the newly emplaced, hot crustal rocks and cold, overlying seawater drives the intense circulation of seawater through the oceanic crust. Hot, altered seawater is eventually shot back out into the sea via black smoker vents. These and other ocean vents are home to thriving communities of organisms, such as tube worms, clams and shrimp. At the base of the vent food chain are chemosynthetic microbes, which derive their energy from chemical reactions associated with the vents rather than from sunlight energy, as do algae and plants living today in the sunlit uppermost surface of the ocean. Support for a black-smoker origin of life comes from the fact that some of the most primitive microbes (archaea) live there today in waters as hot as 113°C.
Is this where life began? Hydro-thermal (hot water) vents, such as this one on the Galápagos Rift mid-ocean ridge, are host to a diverse community of organisms (mostly white-and-red tube worms in the photo above). The vibrant vent community ultimately exists from the energy available from chemical reactions. Some of these reactions are expressed in the precipitation of the dark sulfide minerals that give ‘black smokers’ their name.
The mid-ocean ridge forms a long, continuous submarine mountain chain (red) along which black smokers and other vent systems occur. Early Earth is likely to have included a mid-ocean ridge with black smokers, with the continents only forming later.
We don’t know how many variations on early life there were before LUCA evolved. In fact, we know very little about LUCA itself, except that it likely included features of the simplest microbes found today residing in the vicinity of deep, dark ocean vents. Wherever and however LUCA first appeared, its descendants soon ventured out to other parts of the ocean and, along the way, evolved into many different types of microorganisms. These, in turn, gradually gave rise to the rich diversity of life we are familiar with today in the sea and on the continents.