Where did Eukaryote development occur ?

[Widdekind]

Is this suggested scenario a plausible explanation ?

 

Darwin's "warm little pond" was a deep water, warm spring, in the Archaean Era

 

1. Basal Eukaryote evolved in deep ocean warm springs (~2.7 Gya)

 

Alexander S. Bradley, in the article "Expanding the Limits of Life" (Scientific American [Dec. 2009 AD], pp. 62-67), argues that the earliest Earth lifeforms, were primitive chemosynthetic microbes, inhabiting highly alkaline warm [~90 degrees C] spring systems, like the "Lost City" formation, found 15 km west of the Mid-Atlantic Ridge, and consisting of Calcium Carbonate chimneys. In complete contrast, to the more well-known "Black Smoker" systems actually inside the Mid-Ocean Ridges -- where the water is hot [~400 degrees C] highly acidic, Sulfide minerals make sooty smoke and dark colored chimneys, and "Lifeforms are indirectly dependent upon energy from the Sun" -- warm spring systems like Lost City host organisms completely independent of Solar energy. Thus, the article argues, the earliest Earth lifeforms likely evolved in locations like Lost City.

 

Indeed, in a chart in the article ("Chart 1"), genetic analysis of all extant Earth organisms strongly suggests, that the Last Common Ancestor (LCA) of all Earth life was a hydrogen consuming microbe (and may have been a methane making microbe as well):

Chart 1

-- Basal

Eukaryotes

diverged from hydrogen consuming,

methanogenic, Archaeobacteria

(

Sci.Amer

. [Dec. 2009 AD], pg. 66)

Now, note, that the basal Eukaryotic line diverged from one of these hydrogen consuming microbial groups (orange). And, this basal Eukaryote, defined by the first evolutionary appearance of a true cell Nucleus, appeared in the Archaean Era (before 2.5 Gya*), and probably by about 2.7 Gya**.

*

(DK Illustrated Visual Guides)

Prehistoric Life

, pg. ~40.

**

N. Lane.

Oxygen

, pg. ~53.

 

In addition, the evolution of the cellular Nucleus may have involved the migration of a Eubacterium into a host Archaeobacterium (see "Chart 2"):

Chart 2

-- Basal, Nucleated Eukaryote may represent the "invasion" of a

(hydrogen consuming, methane making, deep ocean warm spring dwelling)

Archaeobacterium

by a neighboring

Eubacterium

([DK Illus. Vis. Guide]

Prehist. Life

, pg. ~40)

.

And, since we strongly suspect, that the host Archaeobacterium-cum-basal-Eukaryote dwelt in the deep ocean, at an alkaline warm spring system, this merger event most likely happened in such a system. Then, referring back to "Chart 1", this location likely indicates, that the invading Eubacterium was something similar to an Aquifex Hydrogenibacter. And, indeed, completely consistent w/ our strengthening suspicions, Aquifex Hydrogenibacter is ancient, thermophilic [heat loving], and chemoautotrophic [sun independent]:

The

Aquifex-Hydrogenobacter

complex belongs to a very early branching order, the

"Aquificales..."

Given the early branching point of the

"Aquificales,"

the characteristics of these organisms support the view that the Last Common Ancestor of existing life was thermophilic and suggest that this ancestor may have fixed carbon chemoautotrophically

*

.

Further still, the order Aquificales stems from the phylum Aquificae, which inhabits warm, alkaline, ocean springs:

The

Aquificae

phylum is a diverse collection of bacteria that live in harsh environmental settings. They have been found in hot springs, sulfur pools, and thermal ocean vents. Members of the genus

Aquifex

, for example, are productive in water between 85 to 95 °C. They are the dominant members of most terrestrial neutral to alkaline hot springs above 60 degrees celsius. They are autotrophs, and are the

primary carbon fixers

in these environments

**

.

Finally, that Aquificae are primary producers, strongly suggests, that the "Nucleation event" stemmed from the incomplete consumption, of the Aquificae Eubacterium, by the Archaeobacterium.

*

http://ijs.sgmjournals.org/cgi/content/abstract/44/4/620

**

http://en.wikipedia.org/wiki/Aquificae

 

 

CONCLUSION:

 

The basal, Nucleated, Eukaryotes evolved in a deep ocean, warm springs system (like Lost City), in the Archaean Era, roughly 2.7 Gya, when a hydrophilic, methanogenic, Archaeobacterium, consumed an Aquificae Eubacterium.

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Is this suggested scenario a plausible explanation ? (REALLY RUSHED ROUGH DRAFT)

 

Mitochondrial merger

 

2. Mitochondrial Eukaryote evolved in shallow water, from merger with an Alpha-proteobacteria (~2 Gya)

 

Eukaryotic mitochondria are closely related to purple, Oxygen-metabolizing, Alpha-proteobacteria*. And, referring back to "Chart 1", not only are (Alpha-)proteobacteria not hydrogen metabolizers -- having long since evolved away from the early alkaline, deep water, warm springs environment of the earliest Earth life -- but neither are any extant Eukaryotes. Moreover, Alpha-proteobacteria are principally phototrophic [light-loving]**. This strongly suggests, that the second major, Mitochondrial, merger event happened somewhere in the shallows of the seas.

*

N.Lane,

ibid

., pg. ~153.

**

http://en.wikipedia.org/wiki/Alphaproteobacteria

 

The first firm fossil evidence of a "[Eukaryotic] organism with a cell containing a nucleus and other organelles" is the Grypania, appearing by by about 1.9 Gya*, and possibly as early as 2.1 Gya**. This comes hard on the heels of the Great Oxygenation Event around 2.3 Gya, after which Earth's atmospheric oxygen concentrations reached a "steady 5-18%" of present levels***. This Great Oxygenation Event set the stage for the evolution of Oxygen metabolizing organisms -- in particular, the pre-Eukaryotic, Alpha-proteobacteria.

*

DK,

ibid

., pg. 56.

**

http://en.wikipedia.org/wiki/Grypania

***

N.Lane,

ibid

., pg. ~70.

 

Alpha-proteobacteria are famous for their symbiotic susceptibilities:

The

Alphaproteobacteria

comprise most phototrophic genera, but also several genera metabolising C1-compounds

(e.g. Methylobacterium spp.),

symbionts

of plants

(e.g. Rhizobium spp.)

and animals, and a group of

pathogens

, the

Rickettsiaceae.

Moreover the precursors of the

mitochondria

of

eukaryotic

cells are thought to have originated from

Rickettsia spp.

in this class. Because of their

symbiotic properties

scientists often use

Alphaproteobacteria

of the genus

Agrobacterium

to transfer foreign DNA into plant genomes and they also have many other biotechnological properties

*

.

And, in particular, the Rickettsia alpha-proteobacteria are "obligate intracellular parasites [whose] survival depends on entry, growth, and replication within the cytoplasm of eukaryotic host cells (typically endothelial cells)"**.

*

http://en.wikipedia.org/wiki/Alphaproteobacteria

**

http://en.wikipedia.org/wiki/Rickettsia

 

 

CONCLUSION:

 

The Mitochondrial merger event happened, roughly 2 Gya, somewhere in the shallows of the seas, where Oxygen concentrations were comparatively high, and when a purple parasitic Rickettsia alpha-proteobacterium infected an evolved basal Eukaryote. This suggested scenario explains "merger 2" in "Chart 2".

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Chloroplast merger

 

2. "Chloroplastal" Eukaryote evolved in shallow water, from another merger, with a Cyanobacterium (~1.2 Gya)

 

Plant Chloroplasts are closely related to photosynthetic Cyanobacteria*. Such Cyanobacteria clearly lived in the sea shallows, likely along the sea shore, alongside stromatolites , which are colonies composed (partially) of such Cyanobacteria(as seen in Shark Bay, Australia). As photosynthesizers, Cyanobacteria are "primary producers", and were the "principal primary producers throughout the Proterozoic Eon (2500-543 Mya**)". Finally, the first fossil evidence, for primitive plants, are algal mats, known by about 1.2 Gya***.

*

N.Lane,

ibid

.

**

http://en.wikipedia.org/wiki/Cyanobacteria

***

http://en.wikipedia.org/wiki/Evolutionary_history_of_plants

 

 

CONCLUSION:

 

The "Chloroplastal"merger event happened, roughly 1.2 Gya, somewhere in the shallows of the seas, surely along a sea shore, when a predatory Mitochondrial Eukaryote incompletely consumed a "principal primary producer" Cyanobacterium. This suggested scenario explains "merger 3" in "Chart 2". These 3 merger events each happened roughly every 700 Myr (a time-scale longer than existence of complex, post-Cambrian Explosion, organisms upon this planet [!!]).

Edited December 23, 2009 by Widdekind
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