The Elements of Evolution – Eukaryotes Evolve

Eukaryotes Evolve

The puzzle of the origin of the eukaryotic cell is extremely complicated. ~ Dutch microbiologist Thijs Ettema

To foster communication among the likeminded and thereby take colonial form, archaic bacteria evolved the genic production of kinases: enzymes which mediate chemical energy flows. This innovation was an essential pre-adaptation to cellular complexity, and to multicellularity. After all, bonding begins with communicating shared interests.

Going through life alone is a struggle which may be eased by partnership with another. Life as a cell isn’t any different.

~2.5 BYA, an archaean host and an endosymbiotic bacterium committed to an everlasting partnership via irreversible specialization; thus arose single-celled eukaryotes, around the time that continents were emerging, and oxygen levels beginning to rise.

As a pre-adaptation, host archaea had invented membrane technology that afforded keeping guests within their own quarters. As a lure for endosymbiosis, archaea were dining on fatty acids and butane – a diet of carbon-based molecules that would have generated nourishing byproducts for partner bacteria.

The concept caught on. A wily single-celled protist endosymbiotically retained a photosynthetic bacterium, giving rise to the plastid which afforded autotrophy. Whence came plants: from green algae which grew determined to make a living on land.

A variety of eukaryotic cell types arose over several hundred million years via waves of endosymbiotic inspiration. All relied upon extensive intracellular communication to be able to live and work together.

Another cell type arose: a complex consumer of other life, requiring a diet of fresh prefabricated amino acids and vitamins. From petite worms all animals evolved.

Protists arose which straddled the demarcation between animals and plants. Protist is more a leftover grouping than a designation.

Diverging from animals 1 BYA, the forebearer of fungus got a strong cell wall and gained great metabolic versatility. It ate ready-made foodstuffs from other organisms, dead or alive.

Many of the major events in the diversification of life owe to species interactions. The consequences: mitochondria and the origin of the eukaryotic cell; chloroplasts and the origin of plants; dinoflagellates and the origin of coral reefs; lichens, mycorrhizae, and rhizobia and the process of terrestrial plant succession; gut symbionts and animal digestion. ~ American evolutionary biologist John Thompson

One impetus to eukaryotic evolution was efficiency. That division of labor is more productive than solitary toil is a universal fact in all social realms, from biofilms to the societies of creatures.

A communication system was necessary for eukaryotic emergence. This required a shared foundation. The answer was DNA.

Viruses were the vehicle for disseminating DNA to all other life and making it the universal standard. The employment of DNA was positively infectious.

Viruses modulate the function and evolution of all living things. ~ American viral ecologist Joshua Weitz et al

It is also quite likely that early pathogens were provocative in the emergence of eukaryotes. From the earliest life to present day, predation drives evolutionary responses.

Viruses have contributed enormously to the communication between cells, and to the appearance of multicellular organisms on Earth. ~ French virologist Felix Rey

At least one virus was instrumental in the origin of fungi. The virus hijacked the ancestor of fungi, seeking to turn it into a virus factory. The fungus that survived selectively incorporated vital viral bits that afforded pliability in development and gave it greater cooperative and parasitic abilities.

Once organisms have a shared communication system, and are thus able to optimize operations, specialization arises, followed inexorably by interdependence; whereupon eukaryotes evolved.

Many unicellular organisms have a colonial form (pluricellularity), often when food becomes scarce, or as a defense. Single-celled green algae exposed to unicellular predators are easy prey. Clumping makes a difference. Small cell colonies offer an ideal trade-off between security from predation and maintaining sufficient surface area for nutrient uptake.

Then next step was labor specialization. Prokaryotic colonial cells are not differentiated, but cells often coordinate to perform different tasks.

Capsaspora is an amoeba with 3 distinct life stages. The demands of solitude and solidarity with others at different stages calls for flexibility. The communication and coordination needed for multicellularity has similar demands. Though single-celled, Capsaspora has capabilities essential for multicellularity. Some of the same genic tricks have been seen in other single-celled organisms.

Spurring Multicellularity

Eukaryotes evolved in a world filled with bacteria and throughout their shared history these two branches of life have developed a complex set of ways to compete and cooperate with each other. ~ American cytologist Rosanna Alegado et al

Multicellularity arose by 2.1 BYA, and independently evolved at least 46 times in all domains of life. The genetic capability was in place well beforehand. Multicellularity was an adaptive response.

Cooperation is wired into cells. ~ American cytologist Thomas Zwaka

Multicellularity was an evolutionary déjà vu to the emergence of eukaryotes. Both were spurred by cellular cooperation and the promise of greater productivity via division of labor. The primary challenge of multicellularity was coordination among cells to optimize efficacy. Shared chemical languages begat the specialization that characterizes and benefits multicellular organisms.

Multicellularity has its challenges, most notably renegades. Cells in a collective must live and die cooperatively.

When cells die in a group, they could poison each other. Instead, dead animal cells provide recyclable raw materials for those still going (autophagy): a tremendous savings for a heterotroph.

Another peril posed by multicellularity is a single cell growing at the expense of others. That hazard still looms with cancer, where cells refuse to play by the rules.

Early multicellular organisms evolved defenses against miscreants. The green algae Volvox programmatically limits the number of times a cell can divide.

That helps reduce the potential for cells to become renegade. ~ American evolutionary biologist Richard Michod

Choanoflagellates are flagellate eukaryotes, existing as free-living single cells or in rosette-shaped colonies. They are the closest cellular relative to metazoa (animals).

Choanoflagellates evolved not long before the origin of animals. ~ American cytologist Nicole King

The bacterium Algoriphagus machipongonensis inspires a unicellular choanoflagellate to divide into a multicellular organism. The individual cells interact as a coordinated cluster. A specific sulfonolipid in the bacterium’s cell membrane provides the biochemical inducement for choanoflagellate division. The process is reminiscent of animal embryology.

Gliding bacteria employ sulfonolipids in their cell membranes to move about. In multicellular eukaryotes, sulfonolipids regulate cell migration and differentiation.

It is likely that endosymbiotic bacteria were instrumental in the evolution of multicellular eukaryotes. This is consistent with the ongoing importance that microbiota play in eukaryotic life.

Pluricellular clusters, such as biofilms, readily form when conditions are favorable. When the situation changes and single cells are again advantageous, these pluricellular congregations go back to more solitary lifestyles.

Things had to change to enforce multicellularity. One of those things was apoptosis: programmed cell death. It seems odd that cellular suicide is beneficial, but higher rates of apoptosis – with dying cells acting as weak links in cellular connectivity – allow cellular clusters to circumvent growth constraints imposed by physical volume and nutrient flow limitations.

Once apoptosis evolved, along with other traits that engendered and enforced cooperative cellular ventures, such as language development and the trust inherent in specialization, the benefits of multicellularity both ratcheted growth prospects and precluded reversibility to single-cell living.

Multicellularity has arisen independently many times. ~ Michael Benton

 Reproduction Isolation

Cell differentiation was a critical step in the evolution of complex multicellular organisms. An early specialization was isolating cells responsible for reproduction.

Most multicellular organisms entrust the propagation of their genes to a select few germline cells amid a sea of non-reproductive somatic cells. With this the fitness of individual cells and the fitness of the entire organism become decoupled.

There are 2 basic cell types in multicellular eukaryotes: germline and soma. The fundamental difference is a reproductive division of labor.

Germline cells have the singular purpose of perpetuating the genomic lineage. Purview of all else belongs to soma. All bodily functions are accomplished by somatic cells.

Genome protection spurred the evolution of multicellular eukaryotes.

Multicellular organisms set aside germ cells to protect their genetic material, letting other cells – the soma – do the dirty work that damages DNA, their genetic building blocks. ~ American biologist Heather Goldsby

The protection essential for viable offspring is ensuring quality mitochondria. Cells can repair a lot of damage, but if the power plants sputter out the cell dies.

Mitochondrial mutations creep in slowly in plants and basal animals, so a germline isn’t needed. Plants generate gametes from pluripotent somatic stem cells.

In contrast, mitochondrial genetic errors can accumulate quickly in active animals, which have a higher metabolic rate and therefore undergo many more division cycles. Hence most bilateral animals sequester a dedicated germline early in development.

Further, mitochondrial genes are only passed to the next generation through the female germline, in the form of large eggs stuffed with thousands of mitochondria. This protects against errors, as eggs undergo fewer replication cycles than other tissue cells.

Single-sex transmission of mitochondrial genes somewhat restricts genetic variation. This is compensated for in mammals by generating far too many egg cells during development and discriminately using a relative few.

Human females are born with over 6 million egg precursor cells (ovarian follicles). 90% of these cells are selectively culled at the start of puberty in a mysterious process known as atresia.

 Complexity

Multicellularity is one of the major transitions that allowed the evolution of large, complex organisms, fundamentally reshaping Earth’s ecology. ~ American evolutionary biologists Eric Libby & William Ratcliff

The thrust of evolution has always been staying alive. For heterotrophs, the first priority is obtaining and processing nutrients; thus evolved systems for motility, foraging, digestion, and control of these and attendant processes.

In optimizing efficacy and evolving to take advantage of local food sources, multicellularity begat complexity as well as diversity. Physical complexity evolved incrementally, with many innovations that might later be abandoned, as a different approach was better suited to the current environment. Energy efficiency, which is its own advantage, often lessens complexity, even as such efficiency is the epitome of intricacy. It is illimitably harder to attribute sophistication than complexity, and so this apter metric of evolutionary quality has not been attempted.

Unless one considers complexity as an accounting exercise, the issue of intricacy is not as simple as moving parts. One may look at material bits and their linkages and conjecture complexity. Appearances are often deceptive, and complexity is exemplary.

As life is defined by energy and information flows, biomechanics misses the critical picture. Every organism is a confluence of matter, energy, and mind. It is impossible to say that the richness or sophistication of our lives is greater than that of microbes, even as the nominal cell count is 10+ trillion to 1.

A crucial factor compelling physical complexity is the sheer challenge of staying alive in a hostile world, where one may readily succumb to predation, including pathogens. The bigger you are, the bigger target you become. Compensation by complication is not the same as sophistication.

Skeletons are quite costly to produce. It’s very difficult to come up with a reason other than defence for why an animal would bother to create a skeleton for itself. ~ English paleobiologist Rachel Wood

 Sponges

We have really humble beginnings. ~ Italian evolutionary biologist David Pisani

Sponges were the forerunner of all complex metazoa. Sponges appear as incomplex animals, without nervous, digestive, or circulatory systems. Their bodies are jelly-like connective tissue (mesohyl) encased in 2 thin layers of cells.

At the cellular level, sponges are not so simple. Unlike other animals, sponges have differentiated cells that can transform into other types. Another novelty arises in the way sponges build their skeletons, employing specialized cells in a complex, coordinated process.

Division of labor by manufacturer, transporter, and cementer cells, and iteration of the sequential mechanical reactions, allows construction of the skeleton as a self-organized biological structure, with the great plasticity in size and shape required for indeterminate growth. ~ Japanese molecular biologist Sohei Nakayama et al

Sponges host a thriving microbiome that helps them digest the microorganisms they ingest, as well as other functions. Over 40% of sponge cells are bacterial.

Despite simple morphology, early sponges had latent genic sophistication. They carried the gene for sex determination (long thought to have evolved much later) and had the genetic bases for nerve conduction that developed in later animals.

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Organ systems arose incrementally in form and function. Jellyfish evolved a gut with a single opening, a ring of muscles, and a nerve net with no central control.

Flatworms have a single opening, but with different sides for intake and elimination. Flatworms also have a definite head-end for their still-meager nervous system.

From there the evolution of digestion proceeded.

As with sponges, endosymbiotic microbes were essential in breaking down chemically complex nutrients which animal cells were unable to process as efficiently.

Socialization became as important as specialization, starting with microbial association. There is some irony that multicellular eukaryotes, even as they became more ‘complex’, have always relied upon prokaryotes that retained relative ‘simplicity’. Such interdependence is the ecology of life.

While new genes and proteins arose with speciation, antecedent genes were conserved: brought forward through eras of genetic and cellular innovation, in case earlier genic knowledge proved useful again. In many cases it did, though often with a twist of adaptive application.