Needless to say, these are very rough guesses, based on evidence of varying reliability. You should always ask yourself "How do they conclude that?" See the info on geological time here. On one widely advertised account the whole thing began with a "Big Bang" some 13.7 billion years ago.
The oldest rocks I know of are dated at 3.8 BYA but the earth must be somewhat older -- most estimates are from 4 to 5 billion years ago (BYA). A recent report in Nature characterizes the birth of Earth:
"When our Sun was born - in a giant supernova explosion some 4.6 billion years ago - a huge shock wave compressed some of the left over gas into tiny clumps of matter. Eventually these coalesced to form the asteroids, comets, moons and planets of today's Solar System. When, and how quickly, this happened has been difficult to calculate. Computer models suggest that the Earth's metal core began to form between 20 million and 30 million years after the explosion. But previous geological evidence put the figure at around 50 million years after it" (Nature, August 29, 2002)
Back in Darwin's time, many scientists considered the earth to be from 20,000 to 2,000,000 years old. Such a "young" earth did not help Darwin and Wallace's arguments that all life evolved from a common ancestral cell. Critics thought this was too short of time to produce the remarkable variety of life via their idea of evolution by variation and natural selection. And it is! Darwin and others sought to speed up the evolutionary process by adhering to Lamarckist ideas on the inheritance of acquired traits, while advocates of supernatural forces pointed to a Biblical dating of Earth at around 6000 years -- coincidentally, perhaps, the age of writing!
While nineteenth century scientists could talk about relative ages of fossil remains and layered rock strata, absolute dates in years awaited twentieth century radioactive dating methods.
first and still predominate form of life were bacteria-like organisms.
Oxygen levels sufficient to sustain advanced life were not available until 2.5 billion ya. Recent work in Nature (2009) attributes this to a drop in nickel levels in seawater that led to a decline in methane-producing microorganisms. (This is just one of millions if not billions of chance events that had to fall just right in order for primates to be where they are today. I'm not worried about aliens!)
The number of species increased, with a few major periods of extinctions, over the next 600 million years. Presumably the more recent species tend to be more complex as the "easy" niches are already taken! Many climatic and geological changes played a role in species change, including changes in atmospheric oxygen (O2) concentration ranging from about 13 to 31%. (Science, v316, 2007, p.557)
This transition, while a striking transformation at the morphological level (fins to four limbs with five digits on each) now appears to be just a tweaking of the existing Hox genes found almost everywhere including insects, fish, reptiles, and mammals. Darwin had speculated that all these creatures with five digits on their 4 limbs could not have happened independently and all must have descended from a common ancestor. The discovery of the developmental control Homeobox genes about 1983 converted many evolutionary agnostics and skeptics.
Continued land movements, climate changes, maybe even asteroid impacts, throughout the last 60 million years result in major species changes. Dinosaurs go extinct and give opportunity for mammals to expand into new niches -- especially the small nocturnal primates spreading into daytime environments and expanding both in species and size.
East Africa, in particular, has undergone geological changes in this period that created a mountainous north-south rift marked by the volcanic Kilimanjaro. This uplift began over 30 million years ago with much of the rift rising several million years ago -- just around the time of major primate evolutionary developments including bipedalism, larger brain, and a shifting balance of meat in their diets. Many significant fossils, including Lucy, have been recovered from this rift region.
A common theme of human evolution is that rapid cooling and drying cycles over the past 5-10 million years served to force rapid adaptations to diverse climates, driving increased brain size and social structure among our primate ancestors.
That chimp ancestor in turn differentiated to the common chimp and the bonobo. Neither is genetically closer to humans but bonobos share perhaps more physical and behaviorial traits perhaps due to their more neotenous development? See EHE, p. 231. Recent gene evolution comparisons with chimps and monkeys suggest many -- not just a few -- genes in our human ancestor were modified, especially those suspected of relating to cognitive abililty. This suggests specific adaptations, not just brain size change, underly human-chimp differences.
(see the overall picture of human evolution) Bipedalism sets the stage for other distinctive human features that follow over the next several million years-- teeth, diet, eye-hand coordination, brain size, and 'recently' - language.
reported (Nature, 1994) indicate individuals perhaps 4 feet tall with evidence
from foramen magnum that they could walk upright and had different arm bones
than today's knuckle-walking apes. No leg or pelvic bones yet found.
A nearly complete skeleton from this period has recently been reported.
About 3 feet tall, Lucy was a probably a mature woman, of a moderately sexually dimorphic species, dimorphism similar to humans . Her brain appears to be about the size of a chimp. There is a recent 2010 claim that the afarensis species may have used tools to butcher meat at 3.4 mya vs 2.4 mya ago (below).
larger brains than today's chimps -- about half that of humans today.
This was probably accompanied by an increase in meat consumption. At some point during the next million years, primate hair was reduced, skin color became fixed, and clothes devised. Recent findings indicate fish consumption at 1.95 mya by h. erectus or other hominin ancestors.
About 1.6 million years ago, a taller, larger brained "homo erectus" perhaps capable of using fire spread from Africa into much of the world, surviving until at recently as 300,000 years ago. Fire would not only provide warmth and safety at night on the ground, but enable cooking which allows access to foods like meat and roots suitable for colder climates. This goes along with smaller teeth and lesser gut size conjectured to result from cooked food. (See Aielo & WHeeler, Wrangham....)
Unequivocal evidence of controlled cooking is absent until maybe 250,000 years ago, though some argue for cooking as early of 790,000 years ago in Israel. (See notes on dietary changes during human evolution.) Brain (and tooth!) size may be a function of food, as well as a predictor of social capabilities and intelligence. Very recently a quite human footprint was uncovered in Kenya (see News, 2/09),
Large brain, technology --fire, spears, axes .. , mark the beginnings of modern humans (perhaps our cousins). Recent reports in the 12 June, 2003 issue of Nature support this"out of Africa" idea, presenting very modern human skulls dated at 160,000 years ago from what is now Ethiopia. Neanderthals and humans share FOXP2 genes dealing with vocalization, as well as nerve fibers needed to ruln the vocal tract. Scientists argue about what this means for origin of language.
South African caves reveal evidence of shellfish eating and use of red pigments 160,000 years ago.
At some point human ancestors greatly reduced body hair growth and invented clothes -- very likely before migrating into colder climates. One recent effort to date this event dated the evolution of body lice which are adapted to live on clothing rather than hair. That date-- the divergence of body lice from hair lice -- has been estimated by Mark Stoneking (2004?) at between 42000 and 72000 years ago.
Is human language such an advantage that it could have been a factor in evolving the human brain and modern humans? (Should we speculate it might be in the social realm--cooperation and teaching ---maybe theory of mind-- that language facilitated?)
Many have speculated that the big change involved the development of culture and evolution of ideas, e.g. Pagels, 2012.
Evidence from Africa to Asia reveals widespread use of stone and wooden tools, fire over 700,000 years ago. Did modern humans evolve gradually from the previous "erectus" populations more or less independently in several places? (This seems implausible now.)
Existing populations evolving into modern humans intermixed with recent more modern African emigrants.
All modern humans recently (<200,000 years?) emigrated from Africa and replaced existing hominids including Neandertals and other "erectus" descendants. (See "Children of Eve" video). Neanderthals are currently believed to be cousins -- not direct ancestors of modern humans. Recently mtDNA extracted from a 100,000 year old Neandertal molar from Belgium confirms the distinct separation of these two species. (Hanni et al. Cell Biology, 2006)
Recent (2008) mtDNA analyses suggest humans lived in Africa for over 100K years before emigrating elsewhere 60K years ago.
Any theory must explain the obvious differences in human features , e.g. characteristic Asian heads and faces. Thus if modern humans only emerged from Africa a few hundred thousand years ago, one must show this is sufficient time for these local variants to emerge. The "multiregional" theory seeks the best of both views with old bodies and new "minds!" This will become critical if older (>250,000 years?) fossils show up with such "local" features. Chinese and other Asian fossils, in particular, may force revisions in current thinking.
The origin of culture is debated--both definitions and dating of cultural artifacts. Shell beads have been dated at 70,0000 years ago and items of wood and fabric would not survive so any estimate is probably an underestimate. And if one considers abstract objects like words and ideas as culture, no artifacts will ever show up.
Did these early humans have language like ours? This is an important, interesting question-- and probably one that cannot be answered with any confidence. It seems quite plausible to me that the basics of human language were established -- hierarchical phrase structure and a certain amount of recursiveness. Yet since each word must have been invented by someone and picked up by the local culture, early humans had a lot of lexicalization (word creation) to do-- and to the extent that words themselves play a role in cognition -- may have had a "dynamic" mentality as word availability increased. Many representational artifacts dating to about 35,000 years ago have been discovered. Some argue that bone fragments dating to 70,000 years ago may have intentional symbolic engravings. It seems fair to say human language was at least in its beginnings if not well underway at this time.
Analysis of ocean sediments and cores from glaciers reveal great temperature changes over the last 130,000 years, resulting in glaciers in Europe and North America, formation of Anarctica, and great changes in sea levels of the Mediteranean, etc. These must have had a huge impact on hominid survival.
For example, the monster volcanic eruption at Lake Toba on the island of Sumatra 73,000 years ago dispersed several thousand times the ash that was expelled at the recent Mount St Helens eruption, causing major cooling and triggering the ice-age. This caused a huge population crash -- some estimates are that only a few thousand humans anywhere survived. If true, we are all decended from those few survivors, inheriting their features whatever they were, that enabled their survival-- (probably lots of luck!). Recent (2010) investigations support this model.
This would explain the restricted variation in human DNA in contrast to that of chimps; put another way, humans are stunningly similar genetically compared with chimps. It may also have forced increased social cooperation among group members for survival -- already necessary at even at the most basic level -- childbirth.
Cro-Magnon culture included language, clothing, hunting weapons, tools, and art, and other representational objects. (While these items may have pre-existed the Cro-Magnons, they transformed and perfected these elements of culture.) These enabled survival in a very harsh climate as well as indicating a rapidly developing culture. (And of course lots of artifacts such as clothing, baskets, non-stone tools have left no traces.) Humans from Asia begin to populate North and South America no more than18,000 years ago according to DNA analyses. (Some have suggested the earliest humans in the Americas were European hunters of marine animals from what is now the coast of Spain and France, the Soltreans, over 19,000 years ago.)
Agriculture makes possible large population growth. It probably brought about the first permanent settlements and subsequent cultural developments. Earliest group eating sites date to about 12k years ago...2
Epidemics and food availability may have contributed to regional gene differences.
dogs (15000BP), goats and sheep (9500BP), catttle (7000BP), horses (5000BP)
Estimates come from Budiansky, S. (1992/1999). The convenant of the wild: Why animals chose domestication. New Haven: Yale University Press.
Estimates of first settlements age vary from 8 to 12, 000 years ago. Seasonal settlements appear earlier but permanent farming villages with homes, garbage dumps, laws, etc in the Jordan valley may be less than 10,000 years old. Just as mental development differs today in rural and urban settings, one can imagine dramatic changes in mental development at this point in human time.
The domestication of grains may have preceded that of animals by a few thousand years. Rice found in Korea is reported to be 15,000 years old. Recently a number of domesticated carbonized figs, dated to be over 11,200 years old were found in the Jordan valley (Science, June,2006)
(see Limber 1982 manuscript reading)
(see Greene, 1959 reading. The terms used by early writer -- apes, monkeys, pygmies, orangs -- may not be consistent with current scientific names and classifications.)
Dutch explorers brought specimens to Holland from the far East.
Tyson described his "pygmy"'s anatomy in great detail.
These much rumored beasts were finally described in detail by Savage and Wyman (1843-47), who also reported ape tool use.
Articulated transformation of species due to inheritance of acquired characteristics. This was shown to be false by Weismann (1885) but took until the early 1900s to be discarded.
Developed our modern conception of variation and natural selection. Dating of evolutiionary events was problemmatical with estimates of the age of earth ranging from the biblical 6000 years, to Buffon's 18th century 'scientific estimate' of 200,000 years. Obviously extensive time was required for the D&W processes to work; Darwin seems to have estimated earth was several hundred million years old and this led to conflicts with physicists (Kelvin) who had lower estimates. This time compression may have been a factor in Darwin's acceptance of Lamarckism. Today's estimates of 4-5 billion years for the earth's age rely on several factors including radioactive properties of meteorites.
These early investigators saw the importance of a comparative, biological and psychological study of primates. Thorndike and Kohler set forth two different conceptions for primate "thought" -- trial and error or insight? In the Russia, the psychologist Nadia Kohts raised a chimp in home -- something other psychologists' families --the Hayes and the Kelloggs -- followed up on. Kohts also demonstrated in a matching task, that chimps color vision was very similar to humans.
Freud, Piaget, Watson, etc.
Every month it seems some new fossil turns up or old ones reanalyzed.
These curious (or is it peculiar) women spent much of their adult lives studying our closest relatives.
Comparative studies of blood, mtDNA, and DNA have revolutionized comparative evolution and dating of speciation.
Recent work continues the research of Yerkes et al. on ape cognition.
The speculation, begun by Descartes, continues despite three decades of research.