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Part A shows a eukaryotic cell. The illustration indicates that, within the nucleus, operational genes were inherited from an ancestral Eubacteria, and informational genes were inherited from an ancestral Archaebacteria. Part B indicates that the outer membrane of Gram-negative bacteria is derived from Archaea, and the inner membrane is derived from Gram-positive bacteria.
The theory that mitochondria and chloroplasts are endosymbiotic in origin is now widely accepted. More controversial is the proposal that (a) the eukaryotic nucleus resulted from the fusion of archaeal and bacterial genomes, and that (b) Gram-negative bacteria, which have two membranes, resulted from the fusion of Archaea and Gram-positive bacteria, each of which has a single membrane.

The nucleus-first hypothesis proposes that the nucleus evolved in prokaryotes first ( [link] a ), followed by a later fusion of the new eukaryote with bacteria that became mitochondria. The mitochondria-first hypothesis proposes that mitochondria were first established in a prokaryotic host ( [link] b ), which subsequently acquired a nucleus, by fusion or other mechanisms, to become the first eukaryotic cell. Most interestingly, the eukaryote-first hypothesis proposes that prokaryotes actually evolved from eukaryotes by losing genes and complexity ( [link] c ). All of these hypotheses are testable. Only time and more experimentation will determine which hypothesis is best supported by data.

 Part A shows the nucleus-first hypothesis. According to this hypothesis, a primary endosymbiotic event resulted in an ancestral eukaryotic cell acquiring a nucleus, and a secondary endosymbiotic event resulted in the acquisition of a mitochondrion. Part B shows the mitochondrion-first hypothesis. According to this hypothesis, the mitochondrion was acquired before the nucleus, but both were acquired by endosymbiosis. Part C shows the eukaryote-first hypothesis. According to this hypothesis, prokaryotes evolved from eukaryotic cells that lost their nuclei and organelles.
Three alternate hypotheses of eukaryotic and prokaryotic evolution are (a) the nucleus-first hypothesis, (b) the mitochondrion-first hypothesis, and (c) the eukaryote-first hypothesis.

Web and network models

The recognition of the importance of HGT, especially in the evolution of prokaryotes, has caused some to propose abandoning the classic “tree of life” model. In 1999, W. Ford Doolittle proposed a phylogenetic model that resembles a web or a network more than a tree. The hypothesis is that eukaryotes evolved not from a single prokaryotic ancestor, but from a pool of many species that were sharing genes by HGT mechanisms. As shown in [link] a , some individual prokaryotes were responsible for transferring the bacteria that caused mitochondrial development to the new eukaryotes, whereas other species transferred the bacteria that gave rise to chloroplasts. This model is often called the “ web of life    .” In an effort to save the tree analogy, some have proposed using the Ficus tree ( [link] b ) with its multiple trunks as a phylogenetic to represent a diminished evolutionary role for HGT.

 Illustration (a) shows the web of life. The base of this web is an ancestral community of primitive cells. This pool of ancestral cells gave rise to the three domains of life. However, because of gene transfer and endosymbiosis events, connections occur between the branches at various points. Thus, eukaryotic chloroplasts and mitochondria originated in bacterial lineages, and archaea and bacteria have exchanged genes.
In the (a) phylogenetic model proposed by W. Ford Doolittle, the “tree of life” arose from a community of ancestral cells, has multiple trunks, and has connections between branches where horizontal gene transfer has occurred. Visually, this concept is better represented by (b) the multi-trunked Ficus than by the single trunk of the oak similar to the tree drawn by Darwin [link] . (credit b: modification of work by "psyberartist"/Flickr)

Ring of life models

Others have proposed abandoning any tree-like model of phylogeny in favor of a ring structure, the so-called “ ring of life    ” ( [link] ); a phylogenetic model where all three domains of life evolved from a pool of primitive prokaryotes. Lake, again using the conditioned reconstruction algorithm, proposes a ring-like model in which species of all three domains—Archaea, Bacteria, and Eukarya—evolved from a single pool of gene-swapping prokaryotes. His laboratory proposes that this structure is the best fit for data from extensive DNA analyses performed in his laboratory, and that the ring model is the only one that adequately takes HGT and genomic fusion into account. However, other phylogeneticists remain highly skeptical of this model.

Illustration shows a ring with the words “pool of primitive prokaryotes” in the middle. Three arrows point outward from the ring, pointing at the three domains, Bacteria, Archaea, and Eukarya, indicating that all three domains arose from a common pool of prokaryotes.
According to the “ring of life” phylogenetic model, the three domains of life evolved from a pool of primitive prokaryotes.

In summary, the “tree of life” model proposed by Darwin must be modified to include HGT. Does this mean abandoning the tree model completely? Even Lake argues that all attempts should be made to discover some modification of the tree model to allow it to accurately fit his data, and only the inability to do so will sway people toward his ring proposal.

This doesn’t mean a tree, web, or a ring will correlate completely to an accurate description of phylogenetic relationships of life. A consequence of the new thinking about phylogenetic models is the idea that Darwin’s original conception of the phylogenetic tree is too simple, but made sense based on what was known at the time. However, the search for a more useful model moves on: each model serving as hypotheses to be tested with the possibility of developing new models. This is how science advances. These models are used as visualizations to help construct hypothetical evolutionary relationships and understand the massive amount of data being analyzed.

Section summary

The phylogenetic tree, first used by Darwin, is the classic “tree of life” model describing phylogenetic relationships among species, and the most common model used today. New ideas about HGT and genome fusion have caused some to suggest revising the model to resemble webs or rings.

Questions & Answers

Is the "growth and maintenance phase" in a cell's life cycle when cell division is about to occur
Somto Reply
what is the common name of Basidiomycetes
Ogechukwu Reply
الاجزاء النباتية لابد من تعقيمها قبل زراعتها في القوارير
whats this?
do you speak arabic?!
what are bio elements
Shahzad Reply
which are present In Body And such elements Have Great role in our Body there are 16 bio elements that maintains human Body but on The basis of amount There are 6 bio elements present in Concen. of 99% and More Valuable And Highly Concen. element is Oxygent with 65 %
how je pollution brought about
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non is pollution brouhgt about
describe the anatomy of cell division
Ivanovic Reply
Complex traits such as height result from 
Ruben Reply
what is the difference between chloroplasts and mitochondria
Nkalubo Reply
chloroplast in plants and bacterial cell ; mitochondria in animal cells
Diagram of a living cell
Eliza Reply
what is cell
A cell is the smallest basic unit of life.
what's biology
Ogochukwu Reply
this is da study of living and non-living thing in an eco-system
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why do plants store carbohydrates in form of starch and not glucose?
Nutty Reply
Describe the structure of starch?
wat is diffusion
water is life!.. Discuss?
Nutty Reply
why do plants store carbohydrates in form if starch not glucose!
study of living thing
Dennis Reply
what is beyond a liveing cell
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Gabriel Reply
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The surgical removal of d spermduct

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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