The evolution of brain size

Human evolution in the direction of Homo sapiens has essentially been couched in terms of an increase in brain size.  Measurements of prehistoric fossil skulls done by anthropologists show the cranial capacity for different prehistoric species of mankind.  The volume of the brain case was measured with no. 8 shot or with mustard seed in the 19th century.  Cranial capacity is larger than the size of the brain that would have been inside it.  For example, the average cranial capacity today is 1400 cc and the average brain weight is 1330 grams.

There was a steady increase in cranial capacity in prehistoric species.  This is a summary of the facts of physical anthropology:

  • Chimpanzee average brain size is 380 cc (cubic centimetres) [included for comparison]
  • Homo primocreatus / Australopithecus 450 to 500 cc
  • Homo habilis 700 cc
  • Homo ignis / Homo erectus / Homo ergaster 800 to 1100 cc
  • Homo ignis antecessor / Homo erectus 1100 cc
  • Homo praecursor heidelbergensis / pekinensis / afarensis / Homo heidelbergensis 1200 cc with a range of 1100 to 1400 cc
  • Homo centralis / Neanderthal range of 1250 to 1750 cc
  • Homo aequalis / Homo sapiens of 100 000 years ago 1500 cc
  • Homo aequalis fossilis Cro Magnon man  1570 cc
  • Homo aequalis / Modern Man  1400 cc

The evolutionary thesis is that as the brain evolves and increases in size, the animal evolves into a human.  The bigger the brain, the more human the creature.

There is, however, a problem with this.  The brain of Neanderthal was larger than the global average today but he was denied the status of being human.  It was said he could only grunt and not talk, no wonder that this brute went extinct when our species came along.

It was later found, with advances in genetics, that we in Europe carry a percentage of Neanderthal genes.  This either means that we are part-animal or that Neanderthal was human after all.

Christians who adopt an evolutionary perspective on creation and an evolutionary origin for humans have more of a problem than most with human evolution.  From a Christian perspective a human person is not an animal and a human person has a soul.  Some Christians have resolved the issue by dropping belief in a human soul, but for those who still believe that to be created in the image of God is to have a soul – at what point did souls enter the human being?

The narrative goes that an ape-man managed to become far enough evolved and get a big enough brain for God to consider this creature to be worthy of being given a soul.  From then on ensouled humans had moral responsibility for their actions.  Ironically they wiped out all the hominids without souls causing them to become extinct.

For me the above question is totally meaningless.  And the answer given has quite horrific implications – firstly that there could be a creature looking just like a human that had no soul and therefore was simply an animal – although you would have no way of distinguishing between the human and the animal just by looking at them.  Secondly, the highest gift of a soul becomes the justification for mass hominidcide, and this is presented as a good thing.

My alternative thesis is that:

  1. There is no threshold brain size for possession of a soul or for being a morally responsible being.
  2. If you have a soul, you are human.  The soul is whole and in every part, there are no part-souls.  Likewise there were never any hominids part-way to being human.
  3. Even with a cranial capacity only marginally above that of apes, a human life is still possible.

Therefore, all prehistoric species that have been classified as ‘hominids’ or ‘hominins’ have, in fact, been human beings adapted to various sets of circumstances that are not found today.  They have all been peoples with culture, and persons with souls.

I had already reached this conclusion when Homo floresiensis was found on the Island of Flores, Indonesia in 2004.  The skeleton found had a cranial capacity of 380 cc and a stature of one metre.  The find was classified as a dwarfed Homo erectus, but the surprise was that it was dated to only 18 000 years old. 

Homo floresiensis is confirmation that a being with a cranial capacity the same as chimpanzees, can, in fact, be human and have lived not long ago.  The human bones were found in a cave with evidence of fire, stone tools and animals that had been hunted.

Humans are human because they have a soul.  Even the most primitive humans had souls so any being looking like a human had a soul.  There was never a situation of Homo sapiens possessing a soul confronting hominids lacking a soul.  Human evolution has brought about changes to the physical body, but not to the fundamental identity of the human being.

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