Human Evolution: From Darwin to Modern Synthesis
Anthropology: Derived from the Greek words anthropos (human being) and logos (study or discourse), it’s the science encompassing human biology and behavior.
Biological Anthropology:
Anatomical studies and the physical transformations of humans throughout their biological development. Its focus is on humanization.
Socio-cultural Anthropology:
The study of humans as organic beings in relation to other living things. It examines different social systems and group behaviors.
Philosophical Anthropology:
Seeks to establish humanity’s place in the world, its origin, and nature. Its aim is to understand human existence.
Theory of Evolution
Three prominent theories exist:
Creationism:
The world and living things were created by God ex nihilo (from nothing).
Fixism:
(Proposed by Linnaeus) Species do not evolve but remain unchanged over time. Originating with Aristotle, it aligned with the creationist interpretation of the Bible, resulting in fixist creationism. Catastrophism (developed by Cuvier) posited that species extinctions resulted from catastrophic events.
Theory of Spontaneous Generation:
This theory asserted that microorganisms originate spontaneously from substrates.
Evolutionary Revolution and Development
The 16th, 17th, and 18th centuries witnessed a physical revolution spearheaded by Copernicus, Galileo, Kepler, and Newton. This led to widespread acceptance of the heliocentric theory, replacing the geocentric model. The 19th century saw a biological revolution driven by Lamarck, Darwin, and Mendel. Lamarck’s Philosophie Zoologique presented the first scientific theory of evolution, culminating in Darwin’s On the Origin of Species. These works expanded the scientific revolution from the physical world to living beings. The prevailing mentality resisted the idea of non-divine origins for living things, creating conflict with the theory of evolution. However, compelling evidence eventually led to the acceptance of biological evolution as a core concept in modern science. Since the late 20th century, evolution’s scope has broadened to encompass cosmic evolution.
Lamarck’s Philosophie Zoologique argued for a divinely created nature, proposing that living things progress from simple to complex forms. He suggested that microorganisms, arising from spontaneous generation, underwent transformations to adapt to their environment. He attributed diversity to changes in living beings’ behavior driven by adaptive needs. Lamarck’s theory was based on two principles:
The Law of Use and Disuse of Organs:
Organs develop or atrophy based on their use or disuse.
The Law of Inheritance of Acquired Characteristics:
These acquired changes are passed to offspring. While false, this theory was revolutionary by attributing the origin of life not to direct divine intervention (as in creationism), but to the need for environmental adaptation. It retained the concept of spontaneous generation.
Natural Selection
Darwin provided substantial evidence supporting evolution as a natural phenomenon. He identified natural selection as the driving force of evolution. His theory can be summarized as follows:
1.
Life exhibits an immense reproductive capacity; living things tend to multiply beyond the available resources.
2.
Individuals within a species possess unique, randomly occurring characteristics.
3.
Individuals with environmentally advantageous traits are more likely to survive and reproduce.
4.
Favorable traits, passed through inheritance, become increasingly common, while unfavorable traits diminish. This process leads to the emergence of new species.
Natural selection ultimately replaced Lamarck’s first law but retained the concept of inheritance of acquired characteristics. Mendel’s laws of inheritance include:
Law of Uniformity:
Crossing two purebred plant strains results in all hybrid offspring.
Law of Segregation:
Crossing these hybrids produces offspring with a 3:1 ratio of dominant to recessive traits.
Law of Independent Assortment:
When crossing two purebred strains differing in multiple characteristics, hereditary factors are transmitted independently. This implies that offspring inherit pairs of hereditary elements (genes) from each parent, transmitted independently and not solely based on lineage.
Genetic Contributions
Two significant genetic theories emerged:
Mutationist Theory:
Advocated by De Vries, this theory proposed that evolution wasn’t slow and gradual but occurred in bursts, without intermediate forms. Mutations were considered the source of species variability.
Gene Theory:
American biologist Thomas Hunt Morgan, a Nobel laureate, conducted research on mutations and discovered the role of chromosomes in gene transmission. His seminal work is The Theory of the Gene.
Synthetic Theory of Evolution
Dobzhansky played a crucial role in reintegrating Darwin’s evolutionary theory with genetics. His Genetics and the Origin of Species is a cornerstone of the synthetic theory of evolution. He demonstrated how small changes in a small fly population could lead to significant changes in a larger offspring population. Mayr suggested that species divergence could also occur rapidly in small, isolated populations. The synthetic theory reformulates evolutionary theory, synthesizing Darwin’s natural selection with Mendel’s laws of inheritance. It posits that genetic variability within a population, arising from mutations and random genetic recombination, is acted upon by natural selection, favoring traits beneficial to the environment. Key achievements of the synthetic theory include:
1.
Identification of DNA as the hereditary material.
2.
Discovery of DNA’s structure by Watson and Crick.
3.
Identification of RNA’s role in genetic information transmission.
4.
Severo Ochoa’s work deciphering the genetic code, enabling human genome sequencing.
The modern synthesis revealed the universality of the genetic code, confirming the interconnectedness of all living beings.
Current Trends in Evolutionary Science
Research in biological sciences has focused on the mechanisms of evolution, leading to two prominent theories: punctuated equilibrium and endosymbiosis. Additionally, research addresses the origin of life.
Punctuated Equilibrium Theory:
A key aspect of Darwin’s theory was gradualism (slow and gradual species evolution), adopted by the synthetic theory. Gould and Eldredge’s punctuated equilibrium theory challenged this gradualism, arguing that the fossil record often contradicts it. Many species remain unchanged for extended periods before suddenly disappearing, giving way to new organisms. This theory views evolution as occurring through rapid and abrupt changes, an idea previously proposed by Mayr but largely ignored.