Atheists typically attribute the complexity of the human brain to natural processes, primarily evolution through natural selection. The theory of evolution explains how species, including humans, have evolved over millions of years through gradual changes in genetic traits. It is important to note that evolution is a well-established scientific theory supported by extensive evidence from various fields such as genetics, paleontology, and comparative anatomy.
The human brain's complexity can be understood as the result of a long process of incremental changes driven by natural selection. Over millions of years, ancestral species underwent genetic variations that occasionally led to small improvements in brain structure and function. These advantageous changes provided certain individuals with better cognitive abilities, enhancing their chances of survival and reproduction. As a result, these advantageous traits were passed on to future generations, leading to the development of increasingly complex brains over time.
Additionally, the human brain's complexity can be seen as an emergent property arising from the interactions of simpler components. The brain consists of billions of interconnected neurons, and its complexity arises from the intricate patterns of neural connections and the dynamic processes happening within the brain.
It's important to remember that complexity does not necessarily imply design by a higher intelligence. Natural processes can lead to remarkable complexity and diversity over vast periods of time, without the need for divine intervention.