Our Story So Far

The birth of the Universe

• About 13.6 billion years ago an event occurred called Big Bang, which unleashed an unimaginably large amount of energy.  This energy was so great that in the immediate aftermath all sorts of exotic matter and antimatter particles formed, many of which were not stable except in these extreme energy conditions.  Very quickly, as measured in the time units that are meaningful to us, this alphabet soup of particles decayed into the range of subatomic particles which are still with us today, both the matter particles and their anti-matter equivalents.  These include both the Fermions, the various matter particles; and the Bosons, the force-carrying particles for the four fundamental forces which permeate the Universe: Electromagnetism, the Strong and Weak Nuclear Forces, and Gravity.
  
• As an aside, it is meaningless to speculate what might have come before Big Bang.  We have no way of knowing, certainly with our current state of knowledge.  It does not help to posit some kind of Prime Mover, though, because this simply leads us down a logical dead-end in which we are forced to ask who or what created the Prime Mover, and who or what created that, and so on indefinitely.
  
• Most of the matter and antimatter particles which emerged from Big Bang collided and were quickly annihilated, but fortunately, for reasons we do not yet understand, there was a slight asymmetry and some matter particles remained – enough to account for the entire mass of the Universe today.

Stars, Galaxies and the Elements of Life

• The Universe started to expand rapidly, and to cool as it did, a process that remains ongoing.  The particles in this early stage formed a kind of spacedust, evenly distributed but crucially with slight fluctuations in density.  Hydrogen and helium atoms abounded, a few lithium atoms existed, but almost no other atoms had yet been formed.  Gradually, the regions in which the spacedust happened to be distributed most densely coalesced into structures which started to experience the effect of gravity, and a positive feedback loop developed whereby the denser they became, the more they curved their local spacetime so that surrounding matter “fell” towards them.  This in turn led to the formation of networks of “filaments”, creating the conditions for the first stars to form, maybe as early as 100 million years after Big Bang.  The same process operating on a wider scale formed the first galaxies, which appeared after about a billion years.
  
• The first stars were super-massive objects, possibly as much as one million times bigger than our Sun.  Because they were so massive, these early stars had a short lifespan, at least in cosmic terms, of just a few million years.  They began to contract because of their own enormous gravity.  This in turn caused the temperature of the gases inside the star to increase, to a surface temperature maybe 17 times hotter than that of our own Sun.  As they collapsed in on themselves, the nuclear fusion processes inside them produced the range of atomic structures we observe in the Universe today – the “heavier elements” or “metals”.  Many of these stars ultimately exploded as supernovae, creating yet heavier atoms and distributing them as particles across the early Universe.  The most massive stars collapsed to form Black Holes, and gravitational forces eventually caused these Black Holes to merge with others in the same vicinity to create even larger Black Holes, such as we observe today at the centre of galaxies, including our own.
  
• The radiation emitted by these stars also ionised the gas remaining in the early Universe.  As the conditions evolved, a second generation of stars was born, from coalescing clouds of the now-ionised gases and spacedust, including some of the particles of heavier elements released by the first generation of stars.  This second generation of stars was typically smaller than the first, partly because of the presence of the heavier elements, which altered the physics governing their size.  These stars emitted radiation in the visible light spectrum, lighting up the Universe in the way we see today.  Over time, these stars also started to “die” (that is, explode as supernovae or collapse into Black Holes), a third generation of stars emerged, richer still in the trace metals; and so the cycle continues.
  
• The most intense period of star and galaxy formation occurred a few billion years after Big Bang.  The process remains ongoing, albeit at a declining rate.  Still, one estimate suggests that just in our own galaxy, the Milky Way, a new star is born on average roughly every year, although this does not happen in a predictable pattern but rather in bursts of activity followed by periods of relative inactivity.

From Starlight to Solar System

• Around 4.6 billion years ago, some spacedust coalesced to form what would become the Solar System, including our Sun and its satellites, among them planet Earth.  Within the first 100 million years of the Earth forming, it experienced a collision with a huge object, possibly as big as the planet Mars, causing the Earth to eject material which then coalesced to form the Moon.  As the Earth gradually cooled from its initial molten state to below the temperature of boiling water, a crust formed, an atmosphere was created by the gravitational effect of Earth’s mass on the gases spewing out of the volcanoes which dominated the early landscape, and moisture was released from the molten rocks which then fell as rain, creating the primitive oceans by about 3.8 billion years ago. 

The Spark of Life

• The earliest undisputed evidence of life on Earth has been dated to 3.5 billion years ago.  Some estimates put the date even earlier, as little as 750 million years after Earth itself was formed.  The very first, simple microorganisms emerged from the primordial soup, that is the early oceans.  We do not yet have a generally-accepted hypothesis for how this life began, although there is broad scientific consensus that a natural explanation exists.  Probably, chemical effects based mainly on the interaction of carbon and water led to the gradual emergence of self-replicating chemical compounds, which evolved into macromolecules, acquired cell membranes (a major innovation) and finally evolved into what we now know as organic life.  It is possible, although certainly not proven, that the process was helped by chemicals brought from Space by meteors, or even that life itself developed somewhere out in Space and was transported to Earth on one, or more, of the meteors which were abundant in this period.

Evolution, Extinction and Survival

• Once they appeared, cell structures grew gradually more complex.  A critical milestone was the evolution of the “eukaryotic” cell, first appearing perhaps 2 billion years ago and thought to be due to the symbiotic union of bacterial and archaeal cells.  Crucially, eukaryotic cells contain a nucleus in which the genetic material is separated from the surrounding cytoplasm, in contrast to the earlier “prokaryotic” cell, a simpler structure which nonetheless continues to prosper to this day in the form of bacteria and viruses.
  
• The evolution of the eukaryotic cell enabled the development of multi-cellular life forms, eventually producing the complex organisms we see in the world today through the 3-step process of Darwinian evolution: “replicate; vary; select”.  The power of this evolutionary algorithm is such that whilst no specific life-form (such as humans) could be predicted, a general progression towards more complex life forms certainly could be, given enough time for the algorithm to do its work, and the absence of environmental shocks sufficient to destroy, or fundamentally impair, the whole system.
  
• Of course, the environment did not remain completely stable but rather changed over time, partly due to changes in the physical conditions and geography of Earth, and partly driven by the different selection pressures exerted by the evolving life forms themselves.  Life forms which were less successful in adapting to this changing competitive landscape declined and eventually became extinct.  Neither Nature nor the evolutionary algorithm are sentimental, and it is estimated that as many as 99.9% of all the species which have existed on Earth to date have become extinct.
  
• The atmosphere of the young Earth contained very little oxygen, comprising mainly hydrogen sulphide, methane and carbon dioxide.  Between 2.4 and 3.3 billion years ago primitive organisms developed the ability to photosynthesise, and thus began a process of enriching the atmosphere with oxygen.  Gradually, maybe around 1.1 billion years ago or so, eukaryotes evolved into the separate “kingdoms” that we recognise today, including plants, animals, fungi and protozoa.  Still these organisms were all tiny, single-celled creatures, until about 580 million years ago when multi-celled organisms started to appear, at least within the plant, animal and fungi “kingdoms”.  A period of intense species proliferation followed, beginning about 540 million years ago and lasting about 50 million years – this is known as the Cambrian Explosion.  This is when most of the phyla (that is, the sub-categories within each “kingdom”) that we observe today first developed, including the first, ocean-dwelling, vertebrates in the form of fish.
  
• Bacteria had probably moved onto land as early as 2.6 billion years ago.  Plants and fungi then started to colonise land close to the ocean edge, possibly a billion years ago in the case of fungi, and maybe 700 million years ago for plants.  These then evolved to adapt to and flourish in their new habitat.  The earliest clear evidence for animals moving onto land is around 450 million years ago.
  
• There followed oscillating periods of proliferation of species, offset by several mass extinctions, mainly driven by changes in climate.  Mammals first appeared about 200 million years ago, “only” 30 million years or so after the first dinosaurs.  These early mammals were small, probably nocturnal, creatures.  They were successful, insofar as they survived.  But mammals only really began to flourish and diversify after the extinction of the dinosaurs, about 65 million years ago, which is to say around 135 million years after mammals themselves had first appeared.  The leading hypothesis for the cause of the sudden extinction of the dinosaurs is a 10-kilometre asteroid which struck the Earth in what is now Mexico, throwing enormous quantities of dust and other matter into the atmosphere and blocking sunlight so that photosynthesis was catastrophically impaired.  Not just the dinosaurs but 75% of all life forms alive at that time became extinct, and life on Earth would surely be very different today had that giant asteroid not struck.
  
• Whatever, mammals began to flourish.  The oldest accepted fossil record shows that primates had clearly diverged from other mammals by around 55 million years ago, although some scholars suggest the split happened much earlier, maybe as long ago as 80 million years, which is to say well before the extinction of the dinosaurs.  Be that as it may, our early primate ancestors had adapted to become tree-dwellers.  Apes diverged from other primates around 21 million years ago, and eventually, around 7 million years ago or so, “hominins”, the ancestors of modern humans, diverged from our common ancestor with chimpanzees and bonobos, in what is now Africa.  For a long time hominins remained essentially tree-dwelling, with a mainly vegetarian diet.  Fairly quickly, it seems, they transitioned from a social structure based on dominance and promiscuity to one favouring pair-bonding and fidelity.  Eventually, maybe around 4 million years ago, hominins came down from the trees, likely in response to climate changes which reduced the amount of forest and created more savannah. 
• The hypothesis is that the loss of rainforests led to a shortage of food, and increased competition with other species for what food there was.  Our ancestors’ response was to range more widely over grassland in search of new food sources.  Bipedalism, that is walking on two legs, developed, the advantage being that it is more economical for covering long distances, and allows better sight across a savannah terrain.  Sweat glands covering the entire skin provided an advantage for movement in hot sun, loss of hair made the evaporation of sweat possible, and greater pigmentation of skin and eyes protected nuclear DNA from higher UV levels.  Diet changed to become omnivorous and specifically to include more meat, making the species more resilient to temporary changes in the availability of particular foods.  This new diet also provided more calories, fuelling larger brains which themselves were required for and in turn enabled novel developments in social behaviour and technology, for example sharing and tool use. 
• Specifically, by 3 million years ago there is some evidence of a modest increase in brain size, to around one-third the size of our modern brains, as well as evidence of the use of rudimentary stone tools.  Around 2.5 million years ago we see the first evidence of primitive stone tools, and also of the hunting of larger game, so that we can infer that sharing food, and possibly other resources such as shelter, had become an established behaviour.  It is also around this time that what we now call the “hunter-gatherer” lifestyle began.  Around 1.5 million years ago the first tear-shaped hand axes appear, and by around 700,000 years ago we see evidence of spears and bows and arrows.  The earliest evidence of systematic use of fire can confidently be dated to around 400,000 years ago, although some researchers speculate that fire may have been used opportunistically by our ancestors well before this time, perhaps as early as 1.5 million years ago.

From Hominins to Human History

• Over time, a variety of different hominin species developed, although it is only our own, homo sapiens, which now survives.  Distinct hominin species included for example homo habilis; homo heidelbergensis, the Denisovans, and the Neanderthals.  There is evidence of Neanderthals specifically from maybe 400,000 years ago; well before the earliest evidence of our own direct ancestors, homo sapiens, which dates back around 200,000 years.  These early homo sapiens had a body design, and brain size, essentially the same as we have today, and once they appear there is evidence of more advanced stone tools, and tantalising clues of local trade networks developing.  Around 150,000 years ago we see the first evidence of what were labelled homo sapiens sapiens, which is to say entirely modern humans, again in Africa.  This distinction was introduced in an era when it was thought that separate species of contemporary human existed, in the form of different “races”.  This subsequently turned out to have no basis in biological reality, and it is no longer clear that there is a biologically meaningful distinction to be made between homo sapiens and homo sapiens sapiens either.  Hereafter we’ll just use homo sapiens for simplicity.
  
• It is generally accepted that the majority of early hominin development occurred in Africa.  Over time, there were a series of “Out of Africa” migrations, and although it seems likely that many of these early populations eventually became extinct or retreated back, eventually of course modern humans colonised most of the planet.  The first significant migration is thought to have happened around 1.8 million years ago, by a hominin species called homo erectus.  Within 100,000 years this species had spread through central Eurasia to east Asia, including what is now China and Indonesia.  There is some evidence for an expansion of homo sapiens as much as 200,000 years ago, early in its existence, and more definite evidence for another around 125,000 years ago.  However these populations eventually went extinct.  The definitive migration, from which the majority of modern humans are descended, occurred some time around 60,000 years ago, reaching Europe around 50,000 years ago.  These people encountered Neanderthals in the Middle East and Europe and lived alongside and even interbred with them for possibly 7,000 years.  Around 40,000 years ago Neanderthals became extinct, as did the population of homo sapiens living in Europe at the time.  It is not clear why this happened but the leading hypothesis these days is that climate fluctuations were likely responsible, replacing earlier theories such as that humans inflicted genocide on Neanderthals.  In any case, by this time the interbred offspring of the homo sapiens population had spread out further in the world, and went on to colonise most of the planet including re-populating Europe.  The consequence is that all modern humans outside Africa have 1.5-2% Neanderthal DNA, and it is possible that this was in fact crucial to our ancestors’ ability to resist novel pathogens which their forebears had not encountered in Africa.
  
• It is not that the hominin lineage enjoyed a smooth unbroken ascent.  On the contrary, it is clear that along the way there were ebbs and flows in the population, including specific “population bottlenecks” during which the species came close to extinction.  For example, genetic research by a team at the Sapienza University of Rome suggests that between 800-900,000 years ago our ancestors’ total population declined to as few as around 1,280 breeding individuals, remaining at roughly that level for as long as 117,000 years.  The relatively low level of diversity within the human gene pool strongly suggests an even more recent population collapse, although there is not yet a consensus explanation for the specifics.  One theory which was popular in the late 1990s but has since been challenged is that the catastrophic explosion of the Lake Toba supervolcano around 74,000 years ago, incidentally a definite historical event and one of the largest known volcanic eruptions in the Earth’s history, precipitated a decline in the global human population to as few as 10,000 individuals, all living in Africa.  Other research suggests the possibility of a bottleneck over a much longer timeframe, with numbers dropping to as low as 2,000 and remaining low for as long as 100,000 years.  Whatever the details, it seems clear that our ancestors came close to extinction on more than one occasion.
  
• Clearly the species survived all the same, and eventually started to flourish.  The earliest known cave paintings date from 35,000 years ago, and the earliest pottery from 30,000 years ago.  Around 25,000 years ago modern humans arrived in the Americas, the last continent to be peopled.  The numbers involved may have been small – for example, one theory proposes that the entire pre-1492 population of the Americas may have descended from as few as 70 individuals who crossed the original land bridge between Asia and North America.  The last Ice Age ended around 13,000 years ago, and the earliest agriculture came soon after, around 12,000 years ago, or around 10,000 BCE, famously originating in the so-called Fertile Crescent in Mesopotamia, and, less famously, independently in as many as ten other places around the world, more or less at the same period.
  
• For those people who embraced it, a number which grew rapidly and eventually became a sizeable majority, agriculture ended the hunter-gatherer lifestyle which had persisted for the preceding 2.5 million years, or possibly more, and ushered in what became an entirely new way of life, for better and in some respects worse.  In particular, agriculture created the possibility of economic surpluses, so that not every member of the group had to rely on their own resources to eat each day.  This opened the door to the division of labour, and eventually to specialisation of various kinds.  Larger social groups and settlements became possible, because there was enough food to feed the population, and because it was no longer necessary to keep moving to wherever the food was.  This in turn created the opportunity, and arguably the need, for hierarchies to develop, to organise and control larger, more complex communities.  There is intriguing evidence of yet another population bottleneck occurring around 5,000 years ago which may relate to hierarchy.  Genetic data suggests that diversity in the male Y chromosome dropped sharply to a level equivalent to a reproduction ratio between men and women of 1:17.  Since there is no suggestion that the number of males declined as a proportion of the population, the inference is that one average only 1 male in 17 successfully reproduced or, said another way, that those males who did reproduce did so with an average of 17 female partners.
  
• The pace and scale of technological change began to increase, for example with the earliest evidence of copper smelting around 5,000 BCE.  The first cities emerged, around 3,500 BCE in Sumeria, in what is now Iraq.  The wheel was invented around the same time, used first in the Tigris-Euphrates valley.  By around 3,000 BCE there is evidence of iron and glass.  This is also when writing starts to emerge, again in Sumeria, apparently for the purpose of recording and perhaps regulating business transactions.  This also marks the point at which recorded history begins, and we will therefore leave the story there; other than to celebrate the remarkable and undeniable, if admittedly solipsistic fact that during this period as in all others since life itself began there existed some specific organisms whose success in reproducing and helping their offspring to survive led in an unbroken chain to – us.

Timeline summary (“chronology of the human brain”)