Human Evolution: From Primates to Modern Humans
Key Figures and Theories in Evolutionary Biology
1. Linnaeus (18th Century): Proposed the binomial nomenclature (genus and species) and taxonomy (classification of living forms). Classified humans as Homo sapiens. This was an extrascientific theory, as he believed God created all beings.
2. Lamarck (19th Century): An evolutionist who posited that living things adapt by modifying their bodies in response to the environment through effort. He believed these changes were inherited by offspring, producing new species. This theory has been proven false.
3. Darwin (19th Century): Proposed the theory of natural selection, stating that offspring are born with random differences. Due to limited environmental resources, those with advantageous traits survive better (struggle for existence) and leave more offspring. Darwin did not understand the causes of these changes or how they were inherited, as he was unaware of Mendel’s work. He proposed that humans descended from some form of primate. Social Darwinism, an interpretation of Darwin’s theory, suggests that the struggle for existence extends to society, the economy, and states.
Phylogeny and Ontogeny
Phylogeny: The evolutionary history of an organism and its taxonomic group. For example, all humans share a history as a group, descending from primate forms. Similarly, all vertebrates and mammals have a shared evolutionary history.
Ontogeny: The embryonic, fetal, and larval development of an individual organism.
Haeckel’s Recapitulation Theory
Haeckel (19th Century): Proposed the recapitulation theory or biogenetic law, stating that “ontogeny recapitulates phylogeny.” He observed that embryos of evolutionarily later species exhibit features of older ancestral animals. For example, embryos of humans, other mammals, and even vertebrates pass through a fish-like stage. Thus, the evolutionary history of all vertebrates is reflected during embryonic development. Haeckel believed that evolutionary changes occur by adding stages at the end of the ontogeny of preexisting ancestors. He incorrectly thought that vertebrates and fish transform into other species in their adult form. Von Baer, an embryologist, clarified that while embryos of higher forms resemble adult ancestral forms, their embryos also resemble each other more closely in earlier developmental stages.
Divergence Theory of Development
The Divergence Theory of Development (DD) refines Haeckel’s ideas: “New species emerge when an embryo deviates from the usual developmental path (now known to be caused by genetic variability).” Embryos are more similar to each other the later the divergence occurred from a common embryo, indicating their phylogenetic relationship. For example, chimpanzee embryos resemble human embryos for a long time, only diverging later. This suggests a common origin and relatively recent divergence (5-6 million years ago), supported by paleontological evidence. The forelimbs of tetrapods are variants of an ancestral model, showing a common embryogenesis until each group’s specific form develops (bird, crocodile, whale, horse, human).
Neoteny
Neoteny: A case of DD where an animal remains in an embryonic, fetal, or larval stage but its gonads mature prematurely, allowing reproduction. This can lead to the emergence of a new form. For example, the first fish may have arisen from primitive chordate larvae that retained their fish-like form, with the notochord becoming the vertebral column in later generations. Neoteny occurs in some amphibians. It is hypothesized that humans may have emerged from an ancestral primate form that was born prematurely or retained immature features.
Paleontology identifies forms close to modern humans, such as Australopithecus and Homo (hominids, distinct from apes but within hominoids), with several species like H. habilis and H. sapiens. Australopithecus and Homo may have resulted from the neotenic transformation of a primate form.
Ethology and Sociobiology
Ethology and Sociobiology: Study behavior from phylogenetic (innate patterns shaped by evolution), functional (survival and reproduction), ontogenetic (learning and modification of innate patterns), and causal (triggering mechanisms) aspects. They examine the genetic bases of behavior, including human behavior, acknowledging it as a result of gene-environment interaction and Darwinian natural selection. Sociobiology specifically studies the biological basis of social behavior, genetics, and related phenomena (aggression, altruism, mate selection, and reproduction).
Development and Growth
Development: The process towards complexity, maturity, and functional form (implying growth at some stage) depends on the genome. We have: a) “closed” contents resulting from phylogeny (e.g., all humans develop a brain, muscles), and b) “open” contents allowing for non-inherited acquisitions and modifications (e.g., further development of skills, intelligence, and muscle mass through exercise). Genetic (heredity) and environmental (medium) factors always influence development. Growth: An increase in size and living mass (through cell and tissue growth via mitosis and cell differentiation, requiring nutrition), measured as weight or body axis length. Growth and development are related but distinct concepts. Population growth is studied using growth curves, representing size for each age. Boys are, on average, about 10 cm taller than girls.
Human Life Cycle Stages
(Note: There are variations in the classification of these stages among different authors.)
- Haploid Stage or Gamete Phase: Egg and sperm.
- Diploid Stage: Begins after fertilization and includes:
- Embryo: Up to 8 weeks, with tissue and organ formation.
- Fetus: Growth (up to 50 cm in 9 months) and maturation until birth.
- Newborn: Up to 4 weeks.
- Breastfeeding: Up to 18 months, ending at weaning.
- Early Childhood: Up to 2 years.
- Late Childhood or Children: Up to 6 years (infant from 4 to 6 years).
- Third Childhood or Boyhood: Up to 10-11 years for girls and 11-12 years for boys.
- Puberty: Arrival of reproductive capacity, around 13-15 years for boys and 15-17 for girls.
- Adolescence: Growth completion, up to 21 years for females and 25 years for males.
- Adult or Mature: Up to 50 years for females and 60 years for males (young adult up to 40).
- Old Age or Senility: From the 60s onwards (longer if in acceptable conditions past 80; otherwise, decrepitude with serious damage).