5.3 - Critical Steps In The Evolution Of Complex And Intelligent Life On Earth

Question 1

Is our sun/solar system typical?

Answer 1

In composition it is typical but the sun is brighter and bigger than >90% of stars in this region of the galaxy. Perhaps small stars are unlikely to form large enough for earth like planets for life to exist.

Question 2

Is our earth typical?

Answer 2

In many ways, the Earth is a typical planet in our solar system and in the larger universe. It orbits a star, has a solid surface, and is composed of rock and metal. However, in other ways, the Earth is unique and exceptional.

• Earth is not typical of all rocky planets (mars, Venus mercury…) nor of large rocky bodies (titan) in our solar system. It may be typical of life bearing planets on whcih observers evolve.

One of the most important factors that makes Earth unique is the presence of liquid water on its surface. Water is essential for life as we know it, and the Earth is the only known planet where liquid water exists on the surface. The presence of water, combined with other factors such as a stable climate and a protective atmosphere, have allowed for the development and evolution of life on Earth.

In addition, the Earth is also unique in its position in the solar system and its distance from the sun. The Earth is located in the so-called “habitable zone” around the sun, where temperatures are suitable for the existence of liquid water. Its position in the solar system also provides it with a stable climate and a protective magnetic field that helps to shield it from harmful solar radiation.

Another unique feature of the Earth is its rich and diverse biosphere, which includes millions of different species of plants, animals, and microorganisms. The complexity and diversity of life on Earth is not found to the same extent on any other planet in our solar system or currently known exoplanets.

Therefore, while the Earth is in many ways a typical planet, it is also unique and exceptional in a number of ways that have allowed for the development and evolution of life.

Question 3

What are observers

Answer 3

The term “observers” typically refers to conscious beings who are capable of observing and experiencing the world around them. This can include humans, animals, and potentially other forms of intelligent life that may exist in the universe.

In the context of discussions about extraterrestrial life, the term “observers” is often used to refer to intelligent beings who may exist on other planets. The search for extraterrestrial life is primarily focused on the search for microbial life, but there is also speculation about the possibility of intelligent life elsewhere in the universe.

The existence of observers on other planets is currently unknown, and the search for extraterrestrial life is an active area of research that involves a range of scientific disciplines, including astronomy, planetary science, and astrobiology. While the possibility of finding intelligent life elsewhere in the universe is intriguing, it is important to approach the search with an open mind and a scientific perspective, and to carefully evaluate any evidence that may suggest the existence of observers on other planets.

Question 4

How is evolution divided into 2 types towards more complex organisms

Answer 4

Evolution can be divided towards more complex organisms into 2 types:

1) The probability of step occurrences is independent of the absolute probabilities of the steps

2) On average, the steps tend to be passed at evenly spaced intervals through evolution

Question 5

Carters critical step model of evolution

Answer 5

Carter’s critical step model of evolution is a hypothesis that proposes a set of critical steps that are necessary for the emergence of intelligent life in the universe. The model was proposed by Brandon Carter, a theoretical astrophysicist, in 1983.

According to Carter’s model, the critical steps necessary for the emergence of intelligent life are:

• The formation of stars: The first step in the emergence of intelligent life is the formation of stars. This creates the necessary conditions for the formation of planets and the emergence of life.
• The formation of planetary systems: The second step is the formation of planetary systems around stars. This creates the potential for habitable planets to form.
• The emergence of life: The third step is the emergence of life on a habitable planet. This requires a number of factors, such as the presence of water, a stable climate, and the presence of organic molecules.
• The evolution of intelligence: The fourth step is the evolution of intelligent life. This requires the development of advanced cognitive abilities, such as language, problem-solving, and tool use.
• Technological civilization: The fifth step is the development of technological civilization. This requires the development of advanced technologies, such as agriculture, industry, and communication.

According to Carter’s model, each of these steps is critical for the emergence of intelligent life, and the failure of any one of them could prevent the emergence of intelligent life in the universe. The model suggests that the emergence of intelligent life may be a rare event, as each of these steps requires a combination of favorable conditions and events to occur.

Question 6

What is the total length of earths habitability?

Answer 6

Earth has been habitable (and inhabited) for about 4 billion years.

But how long the earth will remain habitable in the future is the question.

The suns luminosity will continue to increase, heating the surface of the planet. Models predict that in about 1 billion years time, this forcing will overwhelm negative feedbacks in earths climate system, and planetary surface temperatures will become too hot for life.

Earths total length of habitability is 5 billion years. Intelligent life has arisen towards the end of this period (4/5th the way through)

Question 7

What did szathmary and Maynard smith (1995) discuss?

Answer 7

They discussed major transitions in evolution that have led to increased complexity in organisms. Most of these are candidates for critical steps.

Major transitions in evolution:

1. Replicating molecules to populations of molecules in compartments
2. Unlinked replicators to chromosomes
3. RNA as gene and enzyme to DNA (genetic code)
4. Prokaryotes to eukaryotes
5. Asexual clones to sexual populations
6. Protists to animals, plants and fungi (cell differentiation/metazoans)
7. Primate societies to human societies (origin of symbolic language)

Question 8

What is an alternative interpretation of critical steps?

Answer 8

We might seek to define the steps as being transitions to more energetic biospheres (involving transformation of both life and global environment).

The steps are then not point events but spread out more in time over 10(8) years for example:

1) Origin of life / prokaryotes
2) Photosynthesis / great oxidation / paleoproterozoic glaciations
3) eukaryotic differentiation / neoproterozoic glaciations/ secondary
4) observers evolve / Anthropocene

Question 9

How is the history of life on earth biased?

Answer 9

The history of life on earth that we observe is biased by the fact that our existence depends on a particular, perhaps unlikely, outcome of evolution

The critical step model is a highly idealised framework offering one way of exploring this problem

If we accept that evolution towards ourselves on earth has (sometimes at least) been placed by unlikely critical steps the model suggests they are likely few in number (4) and very roughly, should be even spaced through earth history

Question 10

What are candidates for the 4 critical steps for the evolution of intelligent life on earth?

Answer 10

1. Origin of life/prokaryotes
2. Photosynthesis / great oxidation / paleoproterozoic glaciations
3. Eukaryotic differentiation / neoproterozoic glaciations / secondary oxidation / cambrain explosiion
4. Observers evolve / Anthropocene

(We look into the evolution of oxygenic photosynthesis and eukaryotes closely as potential critical steps)

Question 11

How does the model help explain that simple life is common in the universe?

Answer 11

It can lend some support to this claim:

If life became established on earth very rapidly, or if many steps are required for first life to evolve, this indicated that prokaryote life is not a critical step. Perhaps it evolved easily, or was seeded onto the earth from elsewhere. In either case, it might then be common elsewhere.

Alternatively if it took 0.8 gyr to evolve, this is compatible with prokaryote life evolving a (single) difficult step.

• Either way, the probability of the first critical step occurring during a planets habitable lifetime is higher than many occurring…

Question 12

How does the model help explain how simple life is common in the universe but complex life is rare?

Answer 12

However, complex life (plants, fungi and particularly animals) are separated from prokaryotes by several (2-4) difficult transitions with low intrinsic probability of occurring. Hence most planets with simple life dont evolve complex life.

Humans are separated from metazoans by one further step. Intelligence is much less common still. The earth is an exceptional planet in that it hosts complex (and furthermore, intelligent) life. It is not typical of planets that bear life.

Question 13

What does oxygenic photosynthesis (op) require?

Answer 13

Chlorophyll pigments = which absorb energy from light

Light harvesting complexes = the most efficient solar cells known

Water splitting centres (WSC) = liberates 4 electrons from H2O

Photosynthesis I and II (PSI,PSII) = generates ATP and NADPH

OP harvests energy from chlorophyll —> uses WSC to convert to electrons —> combines PSI and PSII pathways to generate chemical energy —> used to fix CO2 to organic matter

Question 14

What is the chlorophyll absorption spectra?

Answer 14

Chlorophyll absorbs in the blue and red (reflecting green back at you)

Chlorophyll is a type of pigment found in plants and other photosynthetic organisms that is responsible for capturing light energy and converting it into chemical energy through photosynthesis. Chlorophyll molecules are able to absorb light in the visible spectrum, with the highest absorption occurring in the blue and red parts of the spectrum.

The absorption spectra of chlorophyll molecules can be visualized using a graph that shows the amount of light absorbed at each wavelength. The absorption spectra of chlorophyll a and chlorophyll b, the two most common types of chlorophyll found in plants, are similar but have some differences in the range of wavelengths they absorb most efficiently.

Question 15

What is a water splitting reaction centre?

Answer 15

It uses the energy from 4 photons to liberate 4 electrons.

The water-splitting reaction center (also known as the oxygen-evolving complex) is a part of the photosynthetic apparatus in plants, algae, and cyanobacteria that is responsible for the conversion of light energy into chemical energy through photosynthesis. It is located in the thylakoid membrane of the chloroplast and is involved in the light-dependent reactions of photosynthesis.

The water-splitting reaction center is a complex of proteins and cofactors that contains a cluster of four manganese ions, one calcium ion, and a tyrosine residue. During photosynthesis, the water-splitting reaction center uses light energy to oxidize water molecules and produce oxygen gas, protons, and electrons. This reaction is a key step in the process of photosynthesis, as it provides the electrons and protons needed to generate ATP and NADPH, the energy carriers used in the subsequent steps of photosynthesis.

The water-splitting reaction center is a highly complex and efficient molecular machine, and its study has important implications for the development of artificial photosynthetic systems that can convert light energy into chemical energy for use in fuel production, carbon dioxide reduction, and other applications. Understanding the structure and function of the water-splitting reaction center is an active area of research in the fields of biochemistry, biophysics, and materials science.

Question 16

Photosystem I and photosystem II

Answer 16

Photosystems of each type are found separately in different anoxygenic photosynthesisers but they are linked together in oxygenic photosynthesis (Cyanobacteria)

Type-I photosystems have linear electron transport and convert NADP to NADPH

Type-II phtosystems have cyclic electron transport and converting ADP into ATP

Question 17

What is the Hill and Bendall ‘Z’ scheme?

Answer 17

Electrons derived from water plus an additional 4 photons —> from electrons used in photosystem II cause energy to be transferred to ATP

A further 4 photons —> from electrons in photosystem I, cause energy to be transferred to NADPH

ATP and NADPH drive the ‘dark reactions’ that transfer another 4 electrons to CO2 making CH2O

Question 18

What is the Calvin-Benson Cycle (dark reaction)

Answer 18

The Calvin-Benson cycle, also known as the dark reaction or the light-independent reaction, is a metabolic pathway that occurs in the chloroplasts of plant cells and some photosynthetic bacteria. It is the second stage of photosynthesis, following the absorption of light by chlorophyll and the generation of ATP and NADPH in the light-dependent reaction/ Z- SCHEME!

The Calvin-Benson cycle is a series of biochemical reactions that use the energy from ATP and the reducing power of NADPH to convert carbon dioxide (CO2) into organic molecules, particularly glucose. The cycle is named after its discoverers, Melvin Calvin and Andrew Benson, who used radioactive isotopes to trace the pathway of carbon through photosynthesis.

The cycle consists of three main stages:

Carbon fixation: CO2 molecules from the atmosphere are incorporated into a five-carbon sugar called ribulose-1,5-bisphosphate (RuBP) by the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase). The step involving the RuBisCo enzyme imparts the 3% discrimination in organic matter.

Reduction: The six-carbon molecule produced in the previous stage is then broken down into two molecules of a three-carbon compound called 3-phosphoglycerate (3PG), which are then converted into glyceraldehyde 3-phosphate (G3P) by the addition of ATP and NADPH.
Regeneration: Some of the G3P produced in the previous stage is used to regenerate the original RuBP molecule, which is necessary for the cycle to continue.
The overall reaction of the Calvin-Benson cycle is:

6CO2 + 18 ATP + 12 NADPH + 12 H2O → C6H12O6 (glucose) + 18 ADP + 18 Pi + 12 NADP+

The Calvin-Benson cycle is essential for the production of organic molecules in photosynthetic organisms and plays a critical role in the global carbon cycle.

Question 19

Why is water used in the Z scheme when it is easier to get electrons from H2, H2S, Fe2+…?

Answer 19

Water is used in the light-dependent reactions of photosynthesis to provide a source of electrons and protons (H+ ions) that are used to generate energy-rich molecules, ATP and NADPH. While it is true that hydrogen ions (H+) are easier to obtain than water molecules, the use of water has several advantages in the process of photosynthesis:

While it is easier to obtain electrons from hydrogen ions, the use of water in the light-dependent reactions of photosynthesis provides a more stable and efficient source of electrons and protons that is readily available to photosynthetic organisms.

Question 20

What is the transitional electron donor in photosynthesis?

Answer 20

The transitional electron donor in photosynthesis is water (H2O). During the light-dependent reactions of photosynthesis, light energy is absorbed by chlorophyll molecules in the thylakoid membranes of chloroplasts. This energy is used to split water molecules into oxygen (O2), hydrogen ions (H+), and electrons (e-). This process is known as photolysis or water-splitting.

Water serves as the transitional electron donor in photosynthesis, providing the electrons and protons needed to produce the energy-rich molecules that power the process of photosynthesis.

Question 21

When did oxygenic photosynthesis evolve?

Answer 21

Oxygenic photosynthesis is believed to have evolved approximately 2.4 to 2.5 billion years ago, during the Great Oxygenation Event (GOE). Before this event, the Earth’s atmosphere was primarily composed of gases such as nitrogen, methane, and carbon dioxide, with very little free oxygen.

The evolution of oxygenic photosynthesis was a significant event in the history of life on Earth, as it led to the gradual buildup of atmospheric oxygen over billions of years. This allowed for the evolution of aerobic organisms, which require oxygen to carry out cellular respiration and obtain energy from organic molecules.

The earliest known evidence of oxygenic photosynthesis comes from stromatolites, which are layered structures formed by microbial communities that are found in ancient rock formations. These structures contain fossilized remains of photosynthetic bacteria known as cyanobacteria, which are thought to have been some of the first organisms to carry out oxygenic photosynthesis.

In summary, oxygenic photosynthesis is believed to have evolved approximately 2.4 to 2.5 billion years ago, during the Great Oxygenation Event, and it played a critical role in shaping the Earth’s atmosphere and the evolution of life on our planet.

Question 22

What evidence is there to show that oxygenic photosynthesis could have been around from as early as 2.7 Ga?

Answer 22

While the majority of evidence points to the evolution of oxygenic photosynthesis occurring around 2.4 to 2.5 billion years ago, there is some evidence that suggests that it may have occurred as early as 2.7 billion years ago.

The evidence that has been put forth in support of an earlier evolution of oxygenic photosynthesis includes:

Carbon isotope ratios: The ratios of different carbon isotopes in ancient rocks can provide clues to the presence of oxygen in the Earth’s atmosphere. Some studies have suggested that there was a small increase in atmospheric oxygen around 2.7 billion years ago, which could have been produced by early oxygenic photosynthetic organisms.

Fossil evidence: Some researchers have suggested that fossilized microbial mats found in ancient rock formations, which have been interpreted as evidence of oxygen-producing photosynthetic bacteria, may date back as far as 2.7 billion years ago.

There is resemblance of microfossils to Cyanobacteria. Biomakers are thought to be unique to Cyanobacteria (or to require O2 in their synthesis)

Question 23

How is the eukaryote evolution an information revolution?

Answer 23

As much more information can be passed to the next generation of eukaryotes due to the increased genome size and gene numbers

Modern analogy = computers can store much more information as time goes on and new technologies are developed.

(However, the basic code in which information is stored and processed in computers has not changed, and much of the fundamental machine level programs remain the same.)

Question 24

What are mitochondria?

Answer 24

• They are the energy factories of eukaryote cells.
• They manage the respiration of food by oxygen, to produce energy.
• There can be from one to thousands in a given cell.
• They are descended from a free living heterotrophic bacterium.

Question 25

What are chloroplasts?

Answer 25

They are the site of the photosynthesis in plants and algae.

They are all ultimately descended from a Cyanobacterium.

Question 26

What is the endosymbiosis origin?

Answer 26

Endosymbiosis is a theory that explains the origin of eukaryotic cells, which are cells that have a nucleus and other membrane-bound organelles. According to the endosymbiosis theory, eukaryotic cells evolved from the symbiotic relationship between two or more different types of prokaryotic cells.

The theory suggests that one type of prokaryotic cell, which was likely a bacterium, was engulfed by another type of prokaryotic cell, which was likely an archaeon. Rather than being digested, the bacterium survived inside the archaeon and developed a mutually beneficial relationship with it. Over time, the two cells became increasingly interdependent and eventually evolved into a single, more complex organism with a nucleus and other membrane-bound organelles.

This idea was originally put forward at the turn of the 20th century and was revived and championed by Lynn Margulis.

Question 27

What is the schematic pathway of steps leading to eukaryotes?

Answer 27

The evolution of eukaryotic cells is a complex process that is still not fully understood, but scientists have proposed several key steps that likely contributed to the emergence of eukaryotic cells. Here is a brief schematic pathway of the major steps leading to eukaryotes:

Endosymbiosis: The first step in the evolution of eukaryotic cells was likely the establishment of endosymbiotic relationships between different types of prokaryotic cells. This process may have involved the engulfment of one type of prokaryotic cell by another, leading to the development of a symbiotic relationship between the two.

Origin of the nucleus: Over time, the prokaryotic cell that was engulfed may have evolved into a specialized organelle such as a mitochondrion or a chloroplast. This allowed the host cell to gain new metabolic capabilities, such as aerobic respiration or photosynthesis. The development of these specialized organelles may have required the evolution of a nuclear envelope, which helped to protect the DNA and regulate gene expression.

Origin of the cytoskeleton: Another key step in the evolution of eukaryotic cells was the development of a cytoskeleton, which is a network of protein filaments that provides structural support and helps to maintain cell shape. The cytoskeleton likely evolved from the protein fibers that were already present in the prokaryotic cells that gave rise to eukaryotes.

Origin of endomembrane system: Eukaryotic cells also have an extensive endomembrane system, which includes the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, and various vesicles and other membrane-bound organelles. This system likely evolved from the infoldings of the cell membrane that occurred during the process of endosymbiosis.

Gene regulation and genome complexity: Eukaryotic cells also have more complex genomes than prokaryotic cells, with many genes organized into linear chromosomes. This likely required the evolution of more sophisticated mechanisms for gene regulation, including epigenetic modifications and the development of transcription factors and other regulatory proteins.

Question 28

What evidence is there for the endosymbiosis theory?

Answer 28

• Mitochondria and chloroplasts are about the same size as prokaryotes.
• They both have a double membrane, remnant of the endosymbiosis even that took them into the cell.
• Genetic studies reveal they both have their own circular chromosomes and their own protein synthesis machinery
• They grow/divide in the cell by their own cycle

Question 29

Is red algae with 4 walls secondary endosymbiosis?

Answer 29

No, the presence of four cell walls in red algae is not related to secondary endosymbiosis. Secondary endosymbiosis is a process by which a eukaryotic cell engulfs and retains another eukaryotic cell, resulting in the establishment of a stable endosymbiotic relationship. This process can occur multiple times, resulting in the evolution of complex eukaryotic lineages such as plants and algae

Question 30

What theories are there to suggest how ends bios is occurred?

Answer 30

There are several theories and lines of evidence that suggest possible mechanisms for how endosymbiosis may have occurred. For example, it is thought that the first endosymbiotic relationship may have involved a bacterium that was capable of photosynthesis being engulfed by a non-photosynthetic host cell.

Over time, the two cells may have developed a symbiotic relationship in which the host cell provided protection and nutrients to the photosynthetic bacterium, while the bacterium provided the host cell with a source of energy in the form of photosynthetic products.

Other examples of endosymbiosis may have involved different types of bacteria being engulfed by a host cell and developing a mutualistic relationship over time. For example, mitochondria are thought to have arisen from the engulfment of a bacterium that was capable of aerobic respiration, which allowed the host cell to generate energy more efficiently. Chloroplasts, on the other hand, are thought to have arisen from the engulfment of a photosynthetic cyanobacterium by a non-photosynthetic host cell.

Question 31

What is the embly and martin paper in 2006?

Answer 31

The Embly and Martin review is a scientific paper published in 2006 that provides a comprehensive overview of the current understanding of the origin and evolution of eukaryotic cells. The paper is titled “Eukaryotic evolution, changes and challenges” and was published in the journal Nature.

In the paper, Embly and Martin discuss various theories and lines of evidence related to the origin and evolution of eukaryotic cells, including endosymbiotic theory, the role of lateral gene transfer, and the evolution of complex cellular structures such as the cytoskeleton and endomembrane system. They also discuss the challenges and controversies associated with studying the evolution of eukaryotes, including the difficulty of inferring ancestral states from modern organisms and the limitations of comparative genomics.

One of the key points of the review is the importance of endosymbiosis in the evolution of eukaryotes. The authors argue that endosymbiosis played a major role in the evolution of eukaryotic cells, allowing them to acquire new metabolic capabilities and develop new structures and functions. They also discuss the potential role of endosymbiosis in the evolution of multicellularity, as the establishment of stable endosymbiotic relationships may have facilitated the development of more complex and cooperative cellular structures.

Overall, the Embly and Martin review is a valuable resource for scientists studying the evolution of eukaryotic cells. It provides a thorough overview of the current state of knowledge in the field, while also highlighting the remaining challenges and uncertainties.

Question 32

How are eukaryotes related to the other domains of life?

Answer 32

Eukaryotes are more closely related to the archae

However, mitochondria and chloroplasts are clearly derived from specific bacteria.

Question 33

Tree of life vs the web of life

Answer 33

Tree of life = Ribosomal RNA divides life into the modern 3 branches model

Web of life = Other genes show evidence for extensive lateral gene transfer of the web of life model.