I bet you have played Lego before. Have you ever wondered what are the 'Lego blocks' of our life? Many of you may know the answer to it - yes, it is our 'cells'. My newly translated article, 'Building Blocks of Life', unveils the mysteries underlying the cells - giving you a fuller picture how our cells work, and how the diverse forms of life are arisen.
Click here to read the translated article in Chinese
Click here to read the original article in English
This article is published by 'Ask A Biologist', Arizona State University. I am a volunteer contributor of the programme.
Evolution is such an amazing process, which gives rises to the current biodiversity. No wonder biologist Theodosius Dobzhansky has said, ‘Nothing in biology makes sense except in the light of evolution.’ My answer to your question is, yes, they are related. And I am going to answer you how this happens in the light of evolution.
From the phylogenetic tree of life, we can see the domain Eukaryota emerges much later than the two other prokaryotic domains. Thus, eukaryotes must have a prokaryote-like ancestor. There are two prominent features that are unique to extant eukaryotes – the presences of (1) nucleus and (2) membrane-bound organelles (mitochondria and chloroplasts are examples). From where do eukaryotes acquire these two things?
When we talk about evolution, we must not neglect the natural environment at the time. Ancient Earth had very little amount of oxygen – of course, the high-energy-making aerobic respiration is not as common as now. There were three types of single-celled prokaryotic organisms: one is a proteobacterium that can make use of oxygen to produce energy (respiration); one is a cyanobacterium that converts light energy to chemical energy (photosynthesis); the last is miserable – it has neither of these abilities. The ‘miserable’ one could be even more miserable – the cyanobacteria were producing oxygen and changing the Earth, but it could not utilize it like the proteobacteria could!
Emergence of First Eukaryote
The ‘miserable’ one now developed infoldings in cell membrane to increase its surface area to volume ratio, possibly because it increased the food intake efficiency to compensate for its lower energy conversion efficiency. The infoldings eventually separated from the cell membrane – forming an endomembrane system, enclosing the nucleoid and genetic materials. This is the first eukaryote (eu, true; karyon, nut; meanings in Greek).
Endosymbiotic Theory (or Symbiogenesis)
There came a very rare chance (well, but if you consider how old the Earth is, it is not surprising at all) – The eukaryote engulfed the aerobic proteobacterium, either as food or parasite, scientists are still not quite sure. Both were lucky, the engulfed bacterium avoided the eukaryote’s digestion (Phew!) and the eukaryote assimilated it as its asset to utilize oxygen (Wow!) – no longer miserable! The proteobacterium is now an endosymbiont in the eukaryotic host. This eukaryote is the ancestor of animals, fungi, and other heterotrophs (food-consuming), and the assimilated proteobacteria become the nowadays mitochondria.
The increasingly oxygen-rich environment selected away other eukaryotes that had not engulfed the aerobe, because clearly the endosymbiotic eukaryote accumulated energy faster and reproduced faster.
At another chance, some eukaryotes took a step further – acquiring the cyanobacteria as endosymbiont. How greedy! But it certainly gained the advantage to produce its own oxygen. This eukaryote is the ancestor of plants, algae, and other autotrophs (food-self-producing), and the assimilated cyanobacteria become the nowadays chloroplasts.
Not only does this whole process explain the emergence of eukaryotes, it also explains why we cannot find a cell that possesses chloroplasts but not mitochondria – because proteobacteria won the race!
Such transversion from acquisition of endosymbionts (individuals living dependently to each other) to assimilation of organelles (dependent cellular part) is first outlined by Russian botanist Konstantin Mereschkowski, as endosymbiotic theory (or symbiogenesis). Many scientists thereafter advance the theory with more evidence.
Evidences of Endosymbiotic Theory
Wait a minute! You may say. ‘How do I know this is true?’
This endosymbiotic process is estimated to occur around 1.5 billion years ago – it is indeed hard to prove its validity. However, there are still some traces of evidence that are detected by scientists to support this testable hypothesis.
First, new mitochondria and chloroplasts have their own genomes not contained in the nuclei – they govern their replication on their own. The cell division process is known as binary fission (many use it and the term ‘amitosis’ interchangeably, but amitosis usually refers to the nucleolar division not involving formation of spindle fibres, and is more frequently referred to certain eukaryotic cells) – which is used solely by prokaryotes.
Second, some membrane proteins and lipids are found exclusively in mitochondria, chloroplasts and prokaryotes – including transport protein porins and membrane lipid cardiolipin.
Third, genomic comparisons suggest a close phylogenetic relationship between these two organelles and their proposed origins (proteobacteria and cyanobacteria).
With more advanced microbiological and genomic studies, endosymbiosis grows from a hypothesis to a sound theory. We are now pretty sure how eukaryotes emerge – but this does not stop scientists from finding solutions of more questions. For instance, biologists utilize mitochondrial DNA to unravel the natural history, and astrobiologists use archaea to find origins of life on Earth and other planets. Scientific inquiry is growing like evolution is.
‘This is a false statement’: is something beyond our logical reasoning? ― A review of Douglas R. Holfstadter’s Gödel, Escher, BachRead Now
‘A good poet will usually borrow from authors remote in time, or alien in language, or diverse in interest.’ ― T.S. Eliot, The Sacred Wood
Douglas R. Holfstadter has made an unprecedented move to bond the mathematician Kurt Gödel, the artist Maurits Corpelis Escher, and the composer Johann Sebastian Bach in the common centre of Gödel’s proof on his incompleteness theorem. This proof is targeting the intrinsic logical looseness and limitations in a formal axiomatic system in mathematics.
In a nutshell, mathematicians have been fancying exploiting a list of axioms which could give us “all of the mathematics”: it has to be a complete system – that any given statement is both provable and disprovable; it also has to be a consistent system – that a statement cannot be both proved true and false at the same time. However, such completeness and consistency are inherently contradictory – for example, ‘This statement does not have any proof in the system of Principia Mathematica (PM)’: if this statement is provable, then PM would be inconsistent (PM is self-referentially contradicting); if this statement is unprovable, then PM would be incomplete (the PM lacks the internal proof for it).
The mathematical ground of Gödel’s proof itself is a stand-alone masterpiece, but it also steps further to reveal the epistemology of any formal system science – theorems are the branch-outs developed from an axiomatic trunk, extending towards the vast space of truth while some being unreachable, at its counterpart, negative axioms provide the basis of all negations of theorems, also leaving some falsehoods unreachable. Coincidently, this reminds me a Chinese counterpart: Yin Yang Theory, and the symbol itself would give you the intuitive thought of its similarity with Gödel’s proof (Figure 1).
The book is precious in the presentation of such rigorous and complex ideas. First, the dialogue at the beginning of each chapter unveils the limitations that we may come across in our daily lives – such opening encores with the dialogues in Plato’s Symposium. It successfully rings a bell among the readers, preparing the laymen to digest the mathematical and logical paradoxes. Second, the author’s imagination is far beyond mathematics alone: he traces back the connections from Escher and Bach with the Gödel’s theorem, offering a highly vibrant repertoire of aesthetics that is engraved in any field of knowledge. For example, Bach’s canons and fugues are often self-referential to deliver ambiguous perceptions to the listeners. In the meanwhile, many of the Escher’s artwork seem unreal and challenge our intuitive perception of space. Third, at the final portion of the book, it brings out a twenty-first century grand challenge of artificial intelligence, and how it may shed light on resolving the complex systems of self-reference, offering valuable insight to the futurism.
This book is definitely a classic that any learners at all stages of knowledge and truth inquiry should read, for the most underlying structure of how we learn and perceive.
Biodiversity loss is one of the top threats in Anthropocene. It is estimated that the current species extinction rate is at 1,000 to 10,000 times the background natural rate (Chivian & Berstein, 2008), because of increasing environmental impacts from anthropogenic activities, including global warming, habitat loss, introduction of exotic species (Thomas et al., 2004). As the extinction of every species potentially leads to the extinction of others in the ecosystem matrix, we may unravel snowballing extinction cascades in the future if proper conservation management is inadequate.
Since last century, many captive breeding programs have been launched to salvage endangered species from imminent extinction (Frankham, 2008). They are vibrant in the highly diversified forms: a university lab, a joint conservation group or a zoo. They have been targeted to serving for a wide range of conservation purposes: (1) the crucial goal in maintaining genetic integrity of threatened species; (2) the pivotal role of translocation and re-introduction programmes that restore wild population (Grueber et al., 2015); (3) educating people of the importance of wildlife conservation; and (4) substantiating in-depth research opportunity for future applications.
Captive management is utterly important when the species can no longer sustain the population itself in the wild. Besides the human activities, natural catastrophes such as strong predation pressure, diseases and food shortage can also endanger wild species to an unsecured population. For example, pink pigeon (Nesoenas mayeri) once experienced a population shock down to 10 individuals, thus is enlisted in IUCN Critically Endangered in 1994. They suffered from introduced invasive species (Swinnerton, 2001) and intense food competition (Jones & Owadally, 1988). The compounding effect of captive breeding and reintroduction have been successful to bring back the survival likelihood. It has been downlisted to Endangered and may become eligible for further downlisting in the future. Captive breeding is beneficial to concentrate the resource and management effort, as compared to in-situ conservation, especially when the population is small and vulnerable to natural chance events. Other successful reintroduction examples include scimitar-horned oryx, Californian condor and golden lion tamarin.
The rationale behind the educational purpose of a captivity such as zoos and marine parks is the created affective connections bridging the nature and visitors. In compared to alienated forms of conservation outreach, such as propaganda and leaflets, the actual experience spending with an animal quickly strengthens the bonding and thus raise a higher compassion to save the endangered animals. When reports find out that there are positive effects of zoos on cognitive and affective characteristics (Luebke et al., 2016), captive management breaks the estrangement between public and wildlife, and affirms the community engagement of conservation.
The success of species conservation depends much on our understanding of their behavioral ecology, habitat and reproductive biology. Captive breeding enables researchers and conservationists to study and experiment the mating success and optimal environmental factors that are most favorable to the species. Especially when certain endangered species is endemic and possesses unique physiology, such as keel-scaled boa in Round Island (Casarea dussumieri) (Bloxam & Tonge, 1986), captive breeding safeguards a better prospecting future of the conservation of the species and its closely related relatives. Further genetic and molecule tools can enable us to minimize inbreeding and maintain the genetic diversity by implementing carefully planned captive management, increasing the chance of successful adaptation to the environment upon reintroduction or translocation (Witzenberger & Hochkirch, 2011).
The three pillars of conservation, education and research in captive management are entwined to conserve the endangered species. However, the effectiveness of captive breeding stirs up some debate as more limitations and inadequacies are untangled.
Captivity may be time consuming but rewarding less, failing to improve the conservation status or mitigating the declining population. For example, the habitat of Hainan gibbons (Nomascus hainanus) is severely fragmented and degraded by infrastructure development (Cawthon Lang, 2005). However, their reproduction biology is sophisticated and discerns great effort. Without a proper mating ground, their lack of territories is a prominent reproductive barrier in the wild (Zhou et al., 2008). Although attempts have been made to breed Hainan gibbons in captivity, all have been failed and the captivity plan is abandoned. Currently there are no Hainan gibbons in captivity (Geissmann, T. & Bleisch, 2008).
Latest findings report that students demonstrate no positive learning outcomes at all (Jensen, 2014). The paper concludes, that zoos fail to nurture proactive conservationists among students and empower the pupils to take ameliorative actions. This research conflicts with the common belief that zoos can function as an important education source. The possible reason behind is the differential management of educational programmes in different places, and the report is locally-targeted. It sheds light on the limitation that different zoos take different approaches in delivering education, while some succeed, some have week efforts.
There are disproportionate efforts allocated to certain species because of an anthropocentric view of conservation effort. A better-known example is the ‘panda-centric conservation’ in which Chinese government has invested tremendous conservation funding for its captive management. Giant panda costs each hosting captivity an average of 2.6 million dollars a year, and the cost can compound to 4 million when cubs are arisen (Warren, 2006). The great financial commitment exerts strong burden to the government, NGO and the public. As a result, ‘flagship species’ is the conservation marketing strategy that attracts public awareness and financial donation. However, it potentially skews the conservation priorities in sole humans’ favour and the detrimental species receive less attention (Ducarme, Luque & Courchamp, 2013). Particularly when the panda is successfully downlisted to Vulnerable in IUCN, the flagship may disappear and poses negative impacts on the attitudes of the conservation stakeholders (Simberloff, 1998).
Captive management is undoubtedly an indispensable tool to preserve a ‘insurance population’ for endangered species. However, the daunting crisis of species extinction must be faced squarely and ex-situ conservation is only the last resort to rescue those species in the edge. In-situ conservation, habitat preservation and better wildlife management must be prioritized and captivity is not an excuse to avoid them.