Evolution of humanity
This clock representation shows some of Earth's major geological time units and definitive events. The Hadean eon represents the time before the fossil record of life on Earth; its upper boundary is now regarded as 4.0 Ga (billion years ago). Other subdivisions reflect the evolution of life; the Archean and Proterozoic are both eons, the Palaeozoic, Mesozoic , and Cenozoic are eras of the Phanerozoic eon.
The three million years Quaternary period, the time of recognizable humans, is too small to be visible at this scale. I repeat, too small to be visible on this scale.
It makes you very humble. It is also clear that the evolution of every being did not take place only in this Quaternary period. Each new generation inevitably takes with its body-mind behaviour from its ancestors.
Let us take a closer look at this short period of the last 3 or 4 million years.
Evolution of Modern Humans
Genetic evidence points to an evolutionary divergence between the lineages of humans and the great apes on the African continent 8–5 million years ago (mya). The earliest fossils considered remains of hominins (members of the human lineage) date to at least 4 mya in Africa; they include the genus Australopithecus and other forms.
The next major evolutionary stage, Homo habilis, inhabited sub-Saharan Africa about 2–1.5 mya. Homo habilis appears to have been supplanted by a taller and more humanlike species, Homo erectus, who lived from c. 1,700,000 to 200,000 years ago, gradually migrating into Asia and parts of Europe.
Between c. 600,000 and 200,000 years ago, Homo heidelbergensis, sometimes called archaic Homo sapiens, lived in Africa, Europe, and perhaps parts of Asia. Having features resembling those of both Homo Erectus and modern humans, Homo heidelbergensis may have been an ancestor of modern humans and also of the Neanderthals, who inhabited Europe and western Asia from c. 200,000 to 28,000 years ago.
Fully modern humans (Homo sapiens) have emerged in Africa only c. 150,000 years ago, perhaps having descended directly from Homo erectus or an intermediate species such as Homo heidelbergensis.
Evolution of Brain size
Throughout human evolution, the brain has continued to expand. The estimated average brain mass of A. afarensis (435 grams), whereas the average brain mass of Home sapiens is 1,350 grams.
The increase appears to have begun with Homo habilis (600 grams), notable for having a small body.
Recent Reduction in brain size
Figure 4 below, shows the cranial capacity in fossil apes and hominins over the last 10 million years.
Brain size remained relatively steady throughout the late Miocene and Pliocene, increasing only slightly in Australopithecus compared with earlier hominins.
However, by 2 million years ago, there was a dramatic increase in the growth rate, coinciding with fossil evidence for the earliest members of the genus Homo. This change point is illustrated by a red vertical dotted line (95% CI shown as a thick pink vertical line).
A second change-point is detected at ∼1.5 million years ago, and the rate of brain size increase remains steady through the Pleistocene and the evolution of Homo sapiens.
During the last 100,000 years, brain size has remained steady in Homo sapiens until a rapid and dramatic change point only 3,000 years ago decreased human brain size at a rate fifty times greater than the previous increases in Pleistocene brain volume.
Each black dot represents an individual fossil skull or osteological specimen.
Why Did Human Brains Decrease in Size?
Scientists drew exciting conclusions based on a study with ants (?!). Why ants?
As you know, ants and human society are examples of eusocial environments: any colonial animal species that live in multigenerational family groups in which most individuals cooperate to aid relatively few reproductive group members.
Studies show that human brain evolution patterns were influenced by collective intelligence, a convergent characteristic of diverse group-living animals. The precise role of social information flow, distribution, and transfer as emergent group properties that may affect brain evolution and neural functioning is unclear.
Large brains may not be required to generate complex behaviour, and brain mosaicism and circuitry—rather than overall size—may be important. Computational models and patterns in some ant clades suggest that group-level cognition may select for reduced brain size and/or adaptive brain size variation. Moreover, complex systems theory predicts that greater social complexity derives from individual simplicity.
Complexity in eusocial insect colony organization may involve selection for either smaller, neurally differentiated worker brains or larger able to offset increased production and operation costs metabolically.
Evolution of speech
It is impossible to assess linguistic competency by observing the insides of reassembled fossil craniums. The apparent cerebral expansion in Homo habilis and Homo rudolfensis may imply a general increase in cognitive abilities, manipulative skills, or other factors besides speech.
A humanoid vocal tract is undetectable in fossils because it comprises only soft tissues and leaves no bony landmarks. Although versatile human speech is reasonably linked to a relatively spacious throat and mobile tongue, the absence of such features is not a compelling reason to deny some form of spoken language in ancestral hominins.
It is argued that articulate human speech is impossible without a lowered voice box and an expanded region above it. If this presumption were true, even Neanderthals would be inept vocally and probably also quite primitive cognitively as compared with Late Paleolithic Homo sapiens populations such as the Cro-Magnons.
Gibbons and great apes do not speak, yet they have throat traits belonging to speech. However, to a lesser degree than humans’. The calls of gibbons are wonderfully varied in pitch and pattern, and if such sounds were broken into discrete bits with consonants, they could emulate words. The same may be said for great apes. Orangutans, chimpanzees, and bonobos have sufficiently mobile lips and tongues; they simply lack neural circuitry for speech.
Historically, all human groups manifest rich symbolically mediated language, religion, and social, political, and economic systems, even in the absence of elaborate material culture.
The demands on the social intelligence of people who live in environments with relatively few artefacts are similar to demands placed upon those who depend upon complex technological gadgets and shelters for comfort.
Consequently, prehistoric Homo sapiens cannot be regarded as cognitively less capable than ourselves, and it is impossible to state which hominin species were “fully human” as symbol users.
Highly likely, the human capacity symbolically to represent feelings, situations, objects, and ideas developed before being commandeered by several bits of intelligence and before it became a boon to vocal communication.
This means that our current focus on cognitive awareness needs to be thoroughly reconsidered. Our body is more than just our mind. How about your body awareness? Switch off your mind and do the following exercise:
Stand with your feet straight ahead and shoulder-width apart. Bring your whole body to one side (e.g. the left side); feel the difference in pressure under your feet.
People with reasonable sensitivity feel very well the pressure difference.
The pressure on the left foot is much greater.
Fine, we will go one step further. Move your body back in the middle and bring your left arm to the side.
What happens to the pressure now? Is there a difference, if so, in which foot?
If you want to increase your body sensitivity and use the non-cognitive knowledge of your body, please have a closer look at my website.