| II.1 Timeline: Evolution of the Brain | |
| Animal Type and Event (ccm = cubic centimeter) | Years Since Appearance |
| Sponges have no nerve cells (neurons), no internal organs, and no means for locomotion. They are the most primitive and earliest multi-cellular animals that evolved from cell colonies. Jellyfish have the first nerve cells that appear in the form of a simple net but with no central nervous system (the brain with a spinal cord). They arose after sponges. Flatworms have the first rudimentary brain. They also have eyespots and the components of both sexes in one individual. | 600,000,000 |
| Fish appear that have a reptilian brain or central nervous system. It is a small brain enclosed in a brainpan and a spinal cord protected by a segmented spinal column (backbone). They are the first vertebrates, a group that would eventually include fishes, amphibians, reptiles, birds, and all mammals, thus, humans. | 550,000,000 |
| Amphibians further develop the reptilian brain. But the cerebellum, that part of the brain which coordinates muscular movement, is a mere connecting band. They evolved from lobe-finned fishes. | 380,000,000 |
| Primitive tetrapods developed from a fish with a two-lobed brain. These vertebrates with four limbs include the birds, reptiles, and mammals. | 365,000,000 |
| Reptiles, during their adaptation to life on land, further developed their brain and it reached its most advanced stage about 80 million years later. | 330,000,000 |
| Mammals,
while adapting to nocturnal life and competing with reptiles,
developed the limbic system* as well as a neocortex** when they first
appeared. They are warm-blooded and offspring are fed with milk secreted
from the female mammary glands. Probably evolved from a group of now extinct
mammal-like reptiles. *Primitive part of the brain near the brain stem that is thought to control emotions, behavior, smell, etc. **The new outer part of the brain. |
225,000,000 |
| Birds have a small cerebral cortex (the gray outer portion of the brain). | 195,000,000 |
| Non-human primates, during their adaptation to diurnal (daylight) living and to a life in trees, developed the neocortex into an early primate brain. | 65,000,000 |
| Earliest hominoids (human-like ancestors), apes like Proconsul Africanus | 23,000,000 |
| Ardipithecus had a brain size of approx. 300 ccm | 5,800,000 |
| Australopithecines had a brain size of 310-530 ccm | 4,200,000 |
| Homo habilis had a brain size of 580-750 ccm | 2,200,000 |
| Homo erectus had a brain size 775-1225 ccm | 1,800,000 |
| Homo heidelbergensis had a brain size of 1100-1200 ccm. Earlier known as Homo sapiens archaic, he is the bridge between Homo erectus and modern humans. Neanderthals (brain size 1300-1500ccm) too were offspring but a separate species, thus, no interbreeding with humans. | 600,000 to 300,000 |
| Modern humans, Homo sapiens sapiens, have a brain of 1300-1400 ccm. Evolved from Homo heidelbergensis, they have about 25,000 genes that share a 98.4% similarity with their closest living evolutionary relative, the chimpanzee. However, the 1.6% dissimilarity took 5 million years to evolve and makes a world of difference. It yielded the higher mental functions, and through them, together with culture, the potential for becoming fully human. | 200,000 to present |