Integral World: Exploring Theories of Everything
An independent forum for a critical discussion of the integral philosophy of Ken Wilber
Ken Wilber: Thought as Passion, SUNY 2003Frank Visser, graduated as a psychologist of culture and religion, founded IntegralWorld in 1997. He worked as production manager for various publishing houses and as service manager for various internet companies and lives in Amsterdam. Books: Ken Wilber: Thought as Passion (SUNY, 2003), and The Corona Conspiracy: Combatting Disinformation about the Coronavirus (Kindle, 2020).

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The Major Milestones of Biological Evolution

Frank Visser / ChatGPT

Me to ChatGPT:

  • During evolution some bacteria have changed into more complex organisms. Yet, bacteria still exist, in abundance. Please explain this paradox, and highlight the major milestones of biological evolution.

Follow-up questions:

  • Nick Lane once wrote that there is an abyss between bacteria and all other forms life. How was this abyss crossed and when did that happen?
  • How did multicellularity evolve and how did it provide survival value?
  • In particular, Lane writes that bacteria hit an evolutionary ceiling: larger specimins take longer to divide, hence smaller ones have an advantage. Besides, larger bacteria have a lower surface-volume ratio and therefore run into breathing problems.
  • Why did complex life evolve at all if bacterial life works just fine, and what are the chances that simple life on other planets - if it exists at all - becomes more complex and even intelligent?
  • In what sense can bacterial life be called "intelligent" in its own kind of way?



Biological evolution is a dynamic process that has unfolded over billions of years, resulting in the remarkable diversity of life on our planet. This essay explores the major milestones of biological evolution, tracing the path from the origin of life to the emergence of complex organisms. It highlights key events and transitions that have shaped the course of evolution and paved the way for the existence of multicellular life forms.

Origin of Life:

The origins of life remain a subject of ongoing scientific investigation, but current theories suggest that life on Earth emerged around 3.5 to 4 billion years ago. The exact mechanisms by which simple organic molecules transformed into self-replicating, metabolizing entities are still the focus of research. However, the primordial soup hypothesis and the Miller-Urey experiment provide insights into the potential chemical processes involved in the formation of the building blocks of life.

Prokaryotes: The Era of Bacteria:

The earliest forms of life were prokaryotes, single-celled organisms lacking a nucleus and other membrane-bound organelles. Bacteria represent the dominant group of prokaryotes and have thrived in a wide range of environments for billions of years. They exhibit remarkable genetic diversity, metabolic versatility, and adaptations to extreme conditions. Bacteria played a crucial role in shaping Earth's early atmosphere and laying the foundation for subsequent evolutionary developments.

Oxygen Revolution and the Rise of Eukaryotes:

One of the most significant milestones in biological evolution was the oxygen revolution, which occurred approximately 2.4 billion years ago. Cyanobacteria, capable of oxygenic photosynthesis, began releasing oxygen as a byproduct, leading to the oxygenation of Earth's atmosphere. This increase in atmospheric oxygen created new ecological opportunities and paved the way for the emergence of eukaryotic organisms.

Endosymbiosis and the Evolution of Eukaryotic Cells:

The evolution of eukaryotic cells marked a major transition in the history of life. It is believed that eukaryotes originated through a process called endosymbiosis, wherein a prokaryotic cell engulfed another cell, forming a symbiotic relationship. This event led to the development of mitochondria, which are thought to have evolved from an ancient endosymbiotic relationship between a host cell and an ancestral prokaryote. The acquisition of mitochondria provided eukaryotes with a more efficient energy-generating system, enabling greater metabolic complexity.


Multicellularity represents a significant milestone in the evolution of life. The transition from unicellular to multicellular organisms allowed for increased complexity, division of labor, and specialization of cell types. The precise mechanisms and timing of this transition are still under investigation, but it is believed to have occurred independently multiple times in different lineages. Examples of early multicellular organisms include algae, sponges, and other simple colonial organisms.

Cambrian Explosion and the Diversification of Complex Life:

The Cambrian Explosion, approximately 541 million years ago, was a period of rapid diversification of multicellular life forms. During this time, the fossil record reveals a burst of novel body plans and ecological adaptations, giving rise to the early ancestors of many major animal groups. This event marked a crucial step in the evolution of complex organisms and set the stage for the subsequent development of diverse ecosystems.

Land Colonization and the Rise of Terrestrial Life:

Around 500 million years ago, organisms began to transition from aquatic to terrestrial environments, leading to the colonization of land. This transition required adaptations such as the development of mechanisms for gas exchange, the evolution of limbs for movement, and strategies for water retention. Plants played a crucial role in facilitating the colonization of land by providing stability, nutrients, and habitats for other organisms. The successful establishment of terrestrial life paved the way for the evolution of diverse ecosystems on land.

Evolution of Vertebrates and the Emergence of Tetrapods:

The evolution of vertebrates, animals with a backbone, represents another major milestone in biological history. Fish, the earliest vertebrates, evolved diverse body forms and adaptations for locomotion and feeding. Over time, some fish species ventured onto land, giving rise to tetrapods, the four-limbed vertebrates. This transition from aquatic to terrestrial life is exemplified by the first amphibians, which had a dual lifestyle and relied on both water and land for survival.

Dinosaurs and the Age of Reptiles:

Dinosaurs dominated terrestrial ecosystems for approximately 165 million years, from the Late Triassic to the end of the Cretaceous period. They evolved into a diverse array of species, exhibiting a wide range of body sizes, ecological roles, and behaviors. Dinosaurs played a crucial role in shaping ecosystems and their extinction paved the way for the rise of mammals and other groups.

Mammalian Radiation and the Emergence of Primates:

Following the extinction of dinosaurs, mammals underwent a period of radiation and diversification. Mammals adapted to various ecological niches and evolved diverse body forms, behaviors, and reproductive strategies. Primates, a group of mammals that includes humans, emerged and developed traits such as grasping hands, forward-facing eyes, and complex social behaviors. These characteristics laid the foundation for the eventual evolution of human intelligence and culture.

Human Evolution and Cultural Development:

The evolution of the Homo genus, including our species Homo sapiens, represents a significant milestone in biological history. Humans evolved unique features, such as an upright posture, tool use, complex language, and advanced cognitive abilities. Cultural development played a pivotal role in human evolution, as it allowed the transmission of knowledge, technology, and social behaviors across generations. This cultural evolution further propelled human adaptability and expansion across diverse environments.


The major milestones of biological evolution illustrate the remarkable journey of life on Earth. From the origin of simple prokaryotes to the emergence of complex multicellular organisms and the development of human intelligence, each milestone represents a critical event in the history of life. Understanding these milestones not only provides insights into our own origins but also highlights the resilience, adaptability, and diversity of life on our planet. By studying the past, we gain a deeper appreciation for the interconnectedness and shared ancestry of all living organisms.

Nick Lane once wrote that there is an abyss between bacteria and all other forms life. How was this abyss crossed and when did that happen?

Nick Lane, a prominent biologist, indeed suggested the existence of an "abyss" between bacteria and all other forms of life. This idea refers to the stark differences in complexity and cellular organization between prokaryotic bacteria and eukaryotic organisms. Crossing this abyss required significant evolutionary developments, particularly through endosymbiosis and the emergence of eukaryotic cells. Let's delve into the process and approximate timing of this critical event.

The crossing of the abyss between bacteria and other forms of life occurred through a process known as endosymbiosis. According to the endosymbiotic theory, proposed by Lynn Margulis in the 1960s, certain organelles found within eukaryotic cells, such as mitochondria and plastids (e.g., chloroplasts), were once free-living bacteria that established a symbiotic relationship with a host cell. Over time, these organelles became integral parts of eukaryotic cells, providing them with new functionalities and enabling the emergence of complex life forms.

The timing of this pivotal event in evolutionary history is a subject of ongoing scientific investigation. While it is challenging to pinpoint the exact moment when endosymbiosis occurred, studies suggest that it likely took place between 1.5 to 2 billion years ago. This estimation is based on various lines of evidence, including molecular phylogenetic analyses, fossil records, and geological data.

Mitochondria, which are believed to have originated from an ancient endosymbiotic event involving an ancestral alphaproteobacterium, played a crucial role in the evolution of eukaryotic cells. They provided eukaryotes with a highly efficient energy production system through oxidative phosphorylation, allowing for increased metabolic complexity and the development of larger, more energy-demanding cells.

The origin of plastids, such as chloroplasts in photosynthetic eukaryotes, involved a separate endosymbiotic event. It is widely accepted that plastids originated from the engulfment and integration of a photosynthetic cyanobacterium by a eukaryotic host cell. This event not only provided the host cell with the ability to harness sunlight and perform photosynthesis but also had profound implications for Earth's biosphere by driving the oxygenation of the atmosphere.

The endosymbiotic events that led to the integration of mitochondria and plastids into eukaryotic cells represent transformative milestones in biological evolution. They enabled eukaryotes to acquire new metabolic capabilities, enhanced cellular complexity, and paved the way for the emergence of multicellular organisms.

In summary, the crossing of the abyss between bacteria and all other forms of life occurred through endosymbiosis, wherein certain bacteria established symbiotic relationships with ancestral eukaryotic cells, eventually becoming mitochondria and plastids. This pivotal event likely took place around 1.5 to 2 billion years ago, ushering in a new era of complexity and setting the stage for the evolution of diverse eukaryotic life forms.

How did multicellularity evolve and how did it provide survival value?

Multicellularity, the ability of organisms to form complex structures composed of multiple cells, is a significant milestone in biological evolution. The evolution of multicellularity involved a series of key adaptations and provided several advantages for survival and ecological success. Let's explore the process of multicellularity's evolution and its associated benefits.

The evolution of multicellularity is believed to have emerged independently multiple times throughout evolutionary history. While the precise mechanisms and timing are still areas of active research, the following are some proposed hypotheses and general trends regarding the evolution of multicellularity:

Colonial Theory:

In this hypothesis, multicellularity initially arose through the aggregation and cooperation of genetically identical cells. These cells, known as "colonial organisms," remain loosely connected and do not possess specialized cell types. Volvox, a green algae, is an example of a colonial organism.

Cell Differentiation:

As multicellularity evolved, cells began to specialize and differentiate, performing distinct functions within the organism. This specialization allowed for greater efficiency and division of labor within the multicellular structure. Simple multicellular organisms, such as sponges, exhibit cellular differentiation.

Cell Adhesion and Communication:

Multicellularity required mechanisms for cells to adhere to one another, maintaining physical cohesion, and communicate effectively. Cell adhesion molecules and signaling pathways evolved to facilitate cell-cell interactions, coordination, and cooperation.

The evolution of multicellularity provided several survival advantages, contributing to the success of multicellular organisms:

Increased Size and Structural Complexity:

Multicellularity allowed organisms to grow larger and develop complex body structures, providing benefits such as enhanced locomotion, defense against predators, and access to new ecological niches.

Division of Labor:

Specialization of cells within a multicellular organism allowed for the division of labor. Different cell types could perform specific functions, increasing overall efficiency and effectiveness. This division of labor enabled organisms to undertake specialized tasks, such as nutrient acquisition, reproduction, defense, or sensory perception.

Improved Resource Utilization:

Multicellular organisms could exploit resources more efficiently by coordinating the uptake, transport, and utilization of nutrients. Specialized cell types, such as those involved in digestion or nutrient absorption, allowed for more efficient resource acquisition.

Enhanced Reproductive Strategies:

Multicellularity facilitated the evolution of more complex reproductive strategies. Organisms with differentiated germ cells could dedicate specific cells solely to reproduction, increasing reproductive efficiency and diversity.

Cooperative Interactions:

Multicellularity promoted cooperation among cells, allowing them to work together for the benefit of the whole organism. This cooperation included cell-cell communication, coordinated movement, and collective defense against external threats.

Increased Adaptability and Evolvability:

Multicellular organisms possess a higher degree of genetic diversity and potential for variation due to sexual reproduction, genetic recombination, and cell specialization. This increased genetic diversity enhances their adaptability to changing environments and facilitates the evolution of novel traits through natural selection.

Overall, the evolution of multicellularity provided survival advantages by enabling larger size, structural complexity, division of labor, improved resource utilization, enhanced reproductive strategies, cooperative interactions, and increased adaptability. These benefits contributed to the ecological success and diversification of multicellular organisms, ultimately shaping the complexity and diversity of life on Earth.

In particular, Lane writes that bacteria hit an evolutionary ceiling: larger specimins take longer to divide, hence smaller ones have an advantage. Besides, larger bacteria have a lower surface-volume ratio and therefore run into breathing problems.

Indeed, Nick Lane has highlighted certain limitations that bacteria face, which can contribute to an evolutionary ceiling in terms of size and complexity. These limitations include slower division rates for larger bacteria and potential respiratory challenges due to lower surface-to-volume ratios. These factors can provide advantages for smaller bacteria and may have influenced the evolution of multicellularity and the emergence of more complex organisms. Let's explore these concepts in more detail.

Division Rates and Size:

As bacteria increase in size, their division rates tend to decrease. This is because larger bacteria require more time and resources to replicate their genetic material and partition it into daughter cells. In contrast, smaller bacteria can divide more rapidly, allowing them to reproduce and adapt to changing environments more quickly.

The slower division rates of larger bacteria can be a disadvantage in environments where rapid replication is essential for survival, such as in the presence of competitors or during transient favorable conditions. Smaller bacteria, with their faster division rates, may have an advantage in exploiting these opportunities.

Surface-to-Volume Ratio and Respiration:

Bacteria rely on diffusion to obtain nutrients and eliminate waste products. The surface-to-volume ratio is a critical factor influencing the efficiency of these processes. As bacteria increase in size, their volume increases more rapidly than their surface area. This leads to a decrease in the surface-to-volume ratio, potentially causing respiratory challenges.

Lower surface-to-volume ratios make it difficult for larger bacteria to exchange gases, nutrients, and waste products effectively through their cell membranes. This limitation can lead to a decrease in metabolic efficiency and a greater reliance on specialized structures or mechanisms to enhance surface area, such as invaginations or infoldings of the cell membrane.

Advantage of Small Size:

Given the limitations of larger bacteria, smaller bacteria may have several advantages:

Rapid Replication:

Smaller bacteria can divide more quickly, allowing them to reproduce and adapt to changing conditions at a faster rate.

Enhanced Nutrient Uptake:

Smaller bacteria have a relatively larger surface area compared to their volume, facilitating more efficient nutrient uptake through their cell membranes.

Enhanced Diffusion:

Smaller bacteria can rely more on passive diffusion for gas exchange, nutrient acquisition, and waste elimination, as the shorter distances involved are more amenable to effective diffusion.

It is important to note that while smaller bacteria may have certain advantages, larger bacteria have their own benefits. For instance, larger bacteria can potentially store more resources, exhibit greater resistance to predation, and have the capacity for more complex metabolic capabilities.

The evolutionary ceiling for bacterial size and complexity, as described by Nick Lane, may have played a role in driving the evolution of multicellularity. By forming multicellular structures, cells could overcome the constraints associated with size, division rates, and surface-to-volume ratios. Multicellularity allowed for division of labor, increased surface area for nutrient exchange, and enhanced adaptability through differentiation of specialized cell types.

In summary, the limitations imposed by slower division rates and lower surface-to-volume ratios in larger bacteria can provide advantages for smaller bacteria. These limitations may have influenced the evolution of multicellularity as a means to overcome these constraints and enable the emergence of more complex organisms with specialized cell types, enhanced surface area, and greater adaptability.

Why did complex life evolve at all if bacterial life works just fine, and what are the chances that simple life on other planets - if it exists at all - becomes more complex and even intelligent?

The evolution of complex life, such as multicellular organisms, occurred due to a combination of various factors and environmental conditions. While bacteria have thrived and diversified on Earth for billions of years, the emergence of complex life forms brought about new possibilities and advantages that facilitated further evolutionary development. As for the chances of simple life on other planets becoming more complex and intelligent, it is a topic of speculation and ongoing scientific inquiry.

Exploiting New Niches and Resources:

Complex life forms, including multicellular organisms, evolved to exploit new ecological niches and resources that were not accessible to bacteria. Multicellularity allowed for specialization, division of labor, and the utilization of diverse habitats and food sources. The evolution of complex life was driven by the availability of new opportunities and the potential for increased efficiency in resource utilization.

Coordinated Responses and Adaptability:

Complex organisms, with specialized cell types and sophisticated communication systems, exhibit coordinated responses to environmental challenges. This allows for adaptability to diverse conditions and the ability to exploit dynamic ecological landscapes. Complex organisms can exhibit behavioral, physiological, and morphological adaptations that enhance their survival and reproductive success.

Evolutionary Innovations:

Complex life forms have evolved a range of novel features and innovations that confer advantages in specific contexts. These innovations include complex sensory systems, advanced locomotion, intricate anatomical structures, and intricate social behaviors. These innovations provide complex organisms with increased capabilities for finding resources, evading predators, and competing for mates, among other advantages.

Emergence of Intelligence:

While intelligence is a complex and multifaceted trait, it has evolved in certain lineages on Earth. Intelligence offers organisms the ability to learn, problem-solve, and adapt to changing environments. In some cases, intelligence has facilitated advanced social behaviors, tool use, and cultural transmission of knowledge, leading to the development of complex societies.

Regarding the chances of simple life on other planets becoming more complex and intelligent, it is difficult to provide a definitive answer due to our limited knowledge. The existence of simple life on other planets is still an open question, and the factors that drive evolutionary complexity and intelligence are complex and multifaceted.

However, it is plausible to consider that similar evolutionary pressures and opportunities on other planets could lead to the emergence of complex life forms. If the conditions are conducive, and if life has a chance to evolve and diversify over long periods, there is a possibility that simple life could undergo a process of increasing complexity and, potentially, the development of intelligence.

The evolution of complex life on Earth serves as a reminder that given the right environmental conditions, evolutionary pressures, and sufficient time, life has the potential to diversify and give rise to increasingly complex forms. Exploring the potential for complex life and intelligence beyond Earth is an exciting area of scientific inquiry, but it requires further investigation and the discovery of suitable environments and evidence of life on other planets or moons.

In what sense can bacterial life be called "intelligent" in its own kind of way?

Bacterial life, while lacking the cognitive abilities associated with higher intelligence, exhibits certain characteristics that can be considered intelligent in their own context. While it's important to note that these behaviors are driven by genetic programming and do not involve conscious decision-making, they demonstrate remarkable adaptive capabilities. Here are some examples:

Sensing and Responding to the Environment:

Bacteria possess various sensory mechanisms to detect and respond to changes in their environment. They can detect chemical gradients, light, temperature, pH, and other environmental cues. In response, bacteria exhibit behaviors such as chemotaxis (movement toward or away from chemical gradients), phototaxis (movement in response to light), and thermotaxis (movement in response to temperature gradients). These responses allow bacteria to navigate their surroundings and locate favorable conditions.

Quorum Sensing and Social Behavior:

Bacteria can communicate and coordinate their activities through a process called quorum sensing. By producing and detecting specific signaling molecules, bacteria can assess population density and initiate collective behaviors. This enables them to act as a cohesive group, such as forming biofilms for enhanced survival, coordinating virulence factors during infection, or synchronizing the release of enzymes for efficient nutrient acquisition.

Antibiotic Resistance and Genetic Adaptation:

Bacteria have the remarkable ability to evolve rapidly in response to selective pressures, including the development of resistance to antibiotics. Through mechanisms such as horizontal gene transfer and genetic mutations, bacteria can acquire or develop resistance genes, enabling them to survive in the presence of antibiotics. This adaptive response demonstrates a form of "intelligence" in the sense that bacteria can genetically adapt to novel challenges and rapidly evolve new traits.

Exploiting Ecological Niches:

Bacteria exhibit impressive metabolic diversity and can occupy a wide range of ecological niches. They can efficiently utilize different energy sources, from sunlight to organic compounds, and thrive in extreme environments, such as deep-sea hydrothermal vents or acidic hot springs. Bacteria can adapt their metabolic pathways and biochemical processes to exploit available resources, demonstrating a high degree of flexibility and adaptability to diverse environments.

Formation of Symbiotic Relationships:

Bacteria engage in symbiotic relationships with other organisms, including mutualistic and pathogenic associations. In mutualistic symbiosis, bacteria provide benefits to their host organism, such as nitrogen fixation in plant roots or aiding digestion in the gut microbiome. Pathogenic bacteria can manipulate host physiology to their advantage, demonstrating a level of sophistication in their interaction with host organisms.

While these behaviors are not driven by conscious thought or decision-making processes, they highlight the ability of bacteria to sense and respond to their environment, communicate, adapt genetically, exploit ecological niches, and engage in symbiotic interactions. These characteristics allow bacteria to survive and thrive in diverse and challenging environments, displaying a form of "intelligence" within their own biological context.

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