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Friday, March 20, 2015

Green chemistry


From Wikipedia, the free encyclopedia

Green chemistry, also called sustainable chemistry, is a philosophy of chemical research and engineering that encourages the design of products and processes that minimize the use and generation of hazardous substances.[1] Whereas environmental chemistry is the chemistry of the natural environment, and of pollutant chemicals in nature, green chemistry seeks to reduce the negative impact of chemistry on the environment by preventing pollution at its source and using fewer natural resources.

As a chemical philosophy, green chemistry applies to organic chemistry, inorganic chemistry, biochemistry, analytical chemistry, physical chemistry and even chemical engineering. While green chemistry seems to focus on industrial applications, it does apply to any chemistry choice. Click chemistry is often cited as a style of chemical synthesis that is consistent with the goals of green chemistry. The focus is on minimizing the hazard and maximizing the efficiency of any chemical choice.

In 2005 Ryōji Noyori identified three key developments in green chemistry: use of supercritical carbon dioxide as green solvent, aqueous hydrogen peroxide for clean oxidations and the use of hydrogen in asymmetric synthesis.[2] Examples of applied green chemistry are supercritical water oxidation, on water reactions, and dry media reactions.

Bioengineering is also seen as a promising technique for achieving green chemistry goals. A number of important process chemicals can be synthesized in engineered organisms, such as shikimate, a Tamiflu precursor which is fermented by Roche in bacteria.

The term green chemistry was coined by Paul Anastas in 1991.[3] However, it has been suggested[4] that the concept was originated by Trevor Kletz in his 1978 paper in Chemistry and Industry where he proposed that chemists should seek alternative processes to those involving more dangerous substances and conditions.[5] The "Zero Effluent Lab Manual" was developed by Thomas Hellman Morton when he was on the faculty at Brown University in the 1970s. The Manual is still available on-line via links at the Hendrix College Green Chemistry site. Also in Chemistry and Industry the solvent free green chemistry version of “The Hajos-Parrish Cyclisation” has been highlighted in 1996 by Professors Andrew B. Holmes and G. Richard Stephenson.[6]

Principles

Paul Anastas, then of the United States Environmental Protection Agency, and John C. Warner developed 12 principles of green chemistry,[7] which help to explain what the definition means in practice. The principles cover such concepts as:
  • the design of processes to maximize the amount of raw material that ends up in the product;
  • the use of safe, environment-benign substances, including solvents, whenever possible;
  • the design of energy efficient processes;
  • the best form of waste disposal: not to create it in the first place.
The 12 principles are:
  1. It is better to prevent waste than to treat or clean up waste after it is formed.
  2. Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.
  3. Wherever practicable, synthetic methodologies should be designed to use and generate substances that possess little or no toxicity to human health and the environment.
  4. Chemical products should be designed to preserve efficacy of function while reducing toxicity.
  5. The use of auxiliary substances (e.g. solvents, separation agents, etc.) should be made unnecessary wherever possible and innocuous when used.
  6. Energy requirements should be recognized for their environmental and economic impacts and should be minimized. Synthetic methods should be conducted at ambient temperature and pressure.
  7. A raw material or feedstock should be renewable rather than depleting wherever technically and economically practicable.
  8. Reduce derivatives – Unnecessary derivatization (blocking group, protection/deprotection, temporary modification) should be avoided whenever possible.
  9. Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.
  10. Chemical products should be designed so that at the end of their function they do not persist in the environment and break down into innocuous degradation products.
  11. Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.
  12. Substances and the form of a substance used in a chemical process should be chosen to minimize potential for chemical accidents, including releases, explosions, and fires.

Trends

Attempts are being made not only to quantify the greenness of a chemical process but also to factor in other variables such as chemical yield, the price of reaction components, safety in handling chemicals, hardware demands, energy profile and ease of product workup and purification. In one quantitative study,[8] the reduction of nitrobenzene to aniline receives 64 points out of 100 marking it as an acceptable synthesis overall whereas a synthesis of an amide using HMDS is only described as adequate with a combined 32 points.

Green chemistry is increasingly seen as a powerful tool that researchers must use to evaluate the environmental impact of nanotechnology.[9] As nanomaterials are developed, the environmental and human health impacts of both the products themselves and the processes to make them must be considered to ensure their long-term economic viability.

Examples

In the statement for the 2005 Nobel Prize for Chemistry for "the development of the metathesis method in organic synthesis," the Nobel Prize Committee states, "this represents a great step forward for 'green chemistry', reducing potentially hazardous waste through smarter production. Metathesis is an example of how important basic science has been applied for the benefit of man, society and the environment."[10] The concept of green pharmacy was developed recently based on similar principles.[11]

Hydrazine

Addressing principle #2 is the Peroxide Process for producing hydrazine without cogenerating salt. Hydrazine is traditionally produced by the Olin Raschig process from sodium hypochlorite (the active ingredient in many bleaches) and ammonia. The net reaction produces one equivalent of sodium chloride for every equivalent of the targeted product hydrazine:[12]
NaOCl + 2 NH3 → H2N-NH2 + NaCl + H2O
In the greener Peroxide process hydrogen peroxide is employed as the oxidant, the side product being water. The net conversion follows:
2 NH3 + H2O2 → H2N-NH2 + 2 H2O
Addressing principle #4, this process does not require auxiliary extracting solvents. Methyl ethyl ketone is used as a carrier for the hydrazine, the intermediate ketazide phase separates from the reaction mixture, facilitating workup without the need of an extracting solvent.

1,3-Propanediol

Addressing principle #7 is a green route to 1,3-propanediol, which is traditionally generated from petrochemical precursors. It can be produced from renewable precursors via the bioseparation of 1,3-propanediol using a genetically modified strain of E. coli.[13] This diol is used to make new polyesters for the manufacture of carpets.

Carbon dioxide as blowing agent

In 1996, Dow Chemical won the 1996 Greener Reaction Conditions award for their 100% carbon dioxide blowing agent for polystyrene foam production. Polystyrene foam is a common material used in packing and food transportation. Seven hundred million pounds are produced each year in the United States alone. Traditionally, CFC and other ozone-depleting chemicals were used in the production process of the foam sheets, presenting a serious environmental hazard. Flammable, explosive, and, in some cases toxic hydrocarbons have also been used as CFC replacements, but they present their own problems. Dow Chemical discovered that supercritical carbon dioxide works equally as well as a blowing agent, without the need for hazardous substances, allowing the polystyrene to be more easily recycled. The CO2 used in the process is reused from other industries, so the net carbon released from the process is zero.

Lactide


Lactide

In 2002, Cargill Dow (now NatureWorks) won the Greener Reaction Conditions Award for their improved method for polymerization of polylactic acid . Unfortunately, lactide-base polymers do not perform well and the project was discontinued by Dow soon after the award. Lactic acid is produced by fermenting corn and converted to lactide, the cyclic dimer ester of lactic acid using an efficient, tin-catalyzed cyclization. The L,L-lactide enantiomer is isolated by distillation and polymerized in the melt to make a crystallizable polymer, which has some applications including textiles and apparel, cutlery, and food packaging. Wal-Mart has announced that it is using/will use PLA for its produce packaging. The NatureWorks PLA process substitutes renewable materials for petroleum feedstocks, doesn't require the use of hazardous organic solvents typical in other PLA processes, and results in a high-quality polymer that is recyclable and compostable.

Carpet tile backings

In 2003 Shaw Industries selected a combination of polyolefin resins as the base polymer of choice for EcoWorx due to the low toxicity of its feedstocks, superior adhesion properties, dimensional stability, and its ability to be recycled. The EcoWorx compound also had to be designed to be compatible with nylon carpet fiber. Although EcoWorx may be recovered from any fiber type, nylon-6 provides a significant advantage. Polyolefins are compatible with known nylon-6 depolymerization methods. PVC interferes with those processes. Nylon-6 chemistry is well-known and not addressed in first-generation production. From its inception, EcoWorx met all of the design criteria necessary to satisfy the needs of the marketplace from a performance, health, and environmental standpoint. Research indicated that separation of the fiber and backing through elutriation, grinding, and air separation proved to be the best way to recover the face and backing components, but an infrastructure for returning postconsumer EcoWorx to the elutriation process was necessary. Research also indicated that the postconsumer carpet tile had a positive economic value at the end of its useful life. EcoWorx is recognized by MBDC as a certified cradle-to-cradle design.

Trans and cis fatty acids

Transesterification of fats

In 2005, Archer Daniels Midland (ADM) and Novozymes won the Greener Synthetic Pathways Award for their enzyme interesterification process. In response to the U.S. Food and Drug Administration (FDA) mandated labeling of trans-fats on nutritional information by January 1, 2006, Novozymes and ADM worked together to develop a clean, enzymatic process for the interesterification of oils and fats by interchanging saturated and unsaturated fatty acids. The result is commercially viable products without trans-fats. In addition to the human health benefits of eliminating trans-fats, the process has reduced the use of toxic chemicals and water, prevents vast amounts of byproducts, and reduces the amount of fats and oils wasted.

Bio-succinic acid

In 2011, the Outstanding Green Chemistry Accomplishments by a Small Business Award went to BioAmber Inc. for integrated production and downstream applications of bio-based succinic acid. Succinic acid is a platform chemical that is an important starting material in the formulations of everyday products. Traditionally, succinic acid is produced from petroleum-based feedstocks. BioAmber has developed process and technology that produces succinic acid from the fermentation of renewable feedstocks at a lower cost and lower energy expenditure than the petroleum equivalent while sequestering CO2 rather than emitting it.[14]

Laboratory chemicals

Several laboratory chemicals are controversial from the perspective of Green chemistry. The Massachusetts Institute of Technology has created the [2] to help identify alternatives. Ethidium bromide, xylene, mercury, and formaldehyde have been identified as "worst offenders" which have alternatives.[15] Solvents in particular make a large contribution to the environmental impact of chemical manufacturing and there is a growing focus on introducing Greener solvents into the earliest stage of development of these processes: laboratory-scale reaction and purification methods. In the Pharmaceutical Industry, both GSK[16][17] and Pfizer[18] have published Solvent Selection Guides for their Drug Discovery chemists.

Legislation

Europe

In 2007, Europe put into place the Registration, Evaluation, Authorisation, and Restriction of Chemicals (REACH) program, which requires companies to provide data showing that their products are safe. This regulation (1907/2006) ensures not only the assessment of the chemicals' hazards as well as risks during their uses but also includes measures for banning or restricting/authorising uses of specific substances. ECHA, the EU Chemicals Agency in Helsinki, is implementing the regulation whereas the enforcement lies with the EU member states. The US Toxic Substances Control Act, passed in 1976, in principle has similar provisions but is not comparable to REACH as to its regulatory effectiveness.

On September 29, 2008 California approved two laws which encourage green chemistry, launching the California Green Chemistry Initiative. The law requires California's Department of Toxic Substances Control to prioritize "chemicals of concern", and puts the burden of testing on the agency rather than industry. The laws were criticized by Paul Anastas, who stated that the laws did not go far enough in encouraging research, education, and industry incentives.[19] The law called for regulations to be in place by January 1, 2011, but universal opposition to the previously proposed regulations rendered that date impossible. Mid October 2012 is the new target date for new draft regulations to be in place to implement the law.[20]

United States

Passed in 1990, the Pollution Prevention Act helped create new approaches for dealing with pollution by preventing problems before they happen.

It has been stated that long-standing weaknesses in the U.S. chemical management program, notably the Toxic Substances Control Act (TSCA) of 1976, discounts the hazardous properties of chemicals relative to their function, price, and performance.[21] The report concludes that these market conditions represent a key barrier to the scientific, technical, and commercial success of green chemistry in the U.S., and that fundamental policy changes are needed to correct these weaknesses.[22]

Education

Many institutions offer courses[23] and degrees on Green Chemistry. Examples from across the globe are Denmark's Technical University,[24] and several in the US, e.g. at the Universities of Massachusetts-Boston,[25] Michigan,[26] and Oregon.[27] A masters level course in Green Technology, has been introduced by the Institute of Chemical Technology, India. In the UK at the University of York[28] University of Leicester, Department of Chemistry and MRes in Green Chemistry at Imperial College London. In Spain different universities like the Universidad de Jaume I[29] or the Universidad de Navarra,[30] offer Green Chemistry master courses. There are also websites focusing on green chemistry, such as the Michigan Green Chemistry Clearinghouse at www.migreenchemistry.org.

Apart from its Green Chemistry Master courses the Zurich University of Applied Sciences ZHAW presents an exposition and web page "Making chemistry green" for a broader public, illustrating the 12 principles.[31]

Scientific journals specialized in green chemistry

Controversy

Following historical analyses of the green chemistry development, there have been green chemistry advocates who see it as an innovative way of thinking. On the other hand, there have been chemists who have argued that green chemistry is no more than a public relations label. In fact, a lot of chemists use the term "green chemistry" independently from the green chemistry paradigm, as proposed by Anastas and Warner. This explains the uncertainty of the scientific status of green chemistry.[32]

Awards

Many scientific societies have created awards to encourage research in green chemistry.
  • Australia’s Green Chemistry Challenge Awards overseen by The Royal Australian Chemical Institute (RACI).
  • The Canadian Green Chemistry Medal.[33]
  • In Italy, Green Chemistry activities center around an inter-university consortium known as INCA.[34]
  • In Japan, The Green & Sustainable Chemistry Network oversees the GSC awards program.[35]
  • In the United Kingdom, the Green Chemical Technology Awards are given by Crystal Faraday.[36]
  • In the US, the Presidential Green Chemistry Challenge Awards recognize individuals and businesses.[37][38]

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Mapping redox switches in cyanobacteria advances use as biofuel


Original link:  https://www.pnnl.gov/science/highlights/highlight.asp?id=3937
 

A research team led by PNNL mapped more than 2,100 sites that may switch redox reactions on and off in cyanobacteria. How these redox switches react in light and darkness will affect cyanobacteria’s use in producing biofuels.
 
Results: Chemical reactions involving reduction and oxidation, or redox, play a key role in regulating photosynthesis in plants and metabolism in animals and humans, keeping things running on an even keel. Now, in a study reported in Molecular & Cellular Proteomics, a team of scientists from Pacific Northwest National Laboratory and Washington University in St. Louis shed light on the role redox plays in cyanobacteria, tiny organisms with the potential to produce a lot of energy. The research team discovered more than 2,100 molecular locations inside a cyanobacterium where an amino acid known as cysteine either switched on or off by redox processes when the cyanobacteria were exposed to light or dark. The work significantly expanded the current repertoire of known redox changes within cyanobacteria.

"Despite the significance of cyanobacteria for future industrial needs, the overall knowledge of molecular site-specific redox changes is still very limited, especially under light conditions," said Dr. Wei-Jun Qian, a PNNL bioanalytical chemist who worked on the study. "This work provides a quantitative and site-specific analysis across the entire complement of proteins in cyanobacteria, giving us a much clearer picture of the kind of conditions cyanobacteria will need to thrive in industrial settings."
 
Why It Matters: Cyanobacteria are increasingly recognized as potential microbial biofactories for producing chemicals and biofuels from solar energy and carbon dioxide. Redox changes within the organism can affect the efficiency of biofuel production. Until this study, scientists did not know the specific cysteine sites within the organism that acted like switches, turning redox on and off, particularly as light changes from daylight to night.
 
Methods: Scientists grew cyanobacteria in the presence of oxygen and continuous light until the organisms were a certain size. They then exposed the cyanobacteria to one of three regimes: continuous light, continuous darkness, or continuous light and a chemical mixture that would hinder the cyanobacteria's ability to photosynthesize. The research team harvested the cyanobacteria and analyzed cells using advanced mass spectrometry technologies developed at PNNL to determine which redox changes were triggered and how.

"The information we gained about how redox changes with light and where the redox switches are located can serve as a unique resource for understanding how cyanobacteria can be used for industrial purposes," said Qian.
 
What's Next? Scientists plan to expand their studies to other types of functional changes within the cyanobacteria. The work will lead to a better understanding of redox mechanisms, their impact on protein functioning, and the effects of environmental changes on cyanobacteria.
 
Acknowledgments
 
Sponsors: The U.S. Department of Energy (DOE) Early Career Research Award, the EMSL Research Campaign project, and the DOE Office of Biological and Environmental Research (BER) Genomic Sciences Program under the PNNL Pan-omics project supported portions of this work. Scientists performed some of the experimental work at EMSL, a BER national scientific user facility at PNNL.
 
Reference: Guo J, AY Nguyen, Z Dai, D Su, MJ Gaffrey, RJ Moore, JM Jacobs, ME Monroe, RD Smith, DW Koppenaal, HB Pakrasi, and W Qian. 2014. "Proteome-wide Light/Dark Modulation of Protein Thiol Oxidation in Cyanobacteria Revealed by Quantitative Site-Specific Redox Proteomics." Molecular & Cellular Proteomics 13(12):3270-3285. DOI: 10.1074/mcp.M114.041160.
 
Research Team: Jia Guo, Ziyu Dai, Dian Stu, Matthew J. Gaffrey, Ronald J. Moore, Jon M. Jacobs, Matthew E. Monroe, Richard D. Smith, David W. Koppenaal, and Wei-Jun Qian, PNNL; and Amelia Y. Nguyen and Himadri Pakrasi, Washington University.
 
Research Area: Biological Systems Science
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Green nanotechnology


From Wikipedia, the free encyclopedia

Green nanotechnology refers to the use of nanotechnology to enhance the environmental sustainability of processes producing negative externalities. It also refers to the use of the products of nanotechnology to enhance sustainability. It includes making green nano-products and using nano-products in support of sustainability.
Green nanotechnology has been described as the development of clean technologies, "to minimize potential environmental and human health risks associated with the manufacture and use of nanotechnology products, and to encourage replacement of existing products with new nano-products that are more environmentally friendly throughout their lifecycle."[1]

Goals

Green nanotechnology has two goals: producing nanomaterials and products without harming the environment or human health, and producing nano-products that provide solutions to environmental problems. It uses existing principles of green chemistry and green engineering[2] to make nanomaterials and nano-products without toxic ingredients, at low temperatures using less energy and renewable inputs wherever possible, and using lifecycle thinking in all design and engineering stages.

In addition to making nanomaterials and products with less impact to the environment, green nanotechnology also means using nanotechnology to make current manufacturing processes for non-nano materials and products more environmentally friendly. For example, nanoscale membranes can help separate desired chemical reaction products from waste materials. Nanoscale catalysts can make chemical reactions more efficient and less wasteful. Sensors at the nanoscale can form a part of process control systems, working with nano-enabled information systems. Using alternative energy systems, made possible by nanotechnology, is another way to "green" manufacturing processes.

The second goal of green nanotechnology involves developing products that benefit the environment either directly or indirectly. Nanomaterials or products directly can clean hazardous waste sites, desalinate water, treat pollutants, or sense and monitor environmental pollutants. Indirectly, lightweight nanocomposites for automobiles and other means of transportation could save fuel and reduce materials used for production; nanotechnology-enabled fuel cells and light-emitting diodes (LEDs) could reduce pollution from energy generation and help conserve fossil fuels; self-cleaning nanoscale surface coatings could reduce or eliminate many cleaning chemicals used in regular maintenance routines;[3] and enhanced battery life could lead to less material use and less waste. Green Nanotechnology takes a broad systems view of nanomaterials and products, ensuring that unforeseen consequences are minimized and that impacts are anticipated throughout the full life cycle.[4]

Current research

Solar cells

One major project that is being worked on is the development of nanotechnology in solar cells.[5] Solar cells are more efficient as they get tinier and solar energy is a renewable resource. The price per watt of solar energy is lower than one dollar.
Nanotechnology is already used to provide improved performance coatings for photovoltaic (PV) and solar thermal panels. Hydrophobic and self-cleaning properties combine to create more efficient solar panels, especially during inclement weather. PV covered with nanotechnology coatings are said to stay cleaner for longer to ensure maximum energy efficiency is maintained.[6]

Nanoremediation and water treatment

Nanotechnology offers the potential of novel nanomaterials for the treatment of surface water, groundwater, wastewater, and other environmental materials contaminated by toxic metal ions, organic and inorganic solutes, and microorganisms. Due to their unique activity toward recalcitrant contaminants, many nanomaterials are under active research and development for use in the treatment of water and contaminated sites.[7][8]
The present market of nanotech-based technologies applied in water treatment consists of reverse osmosis, nanofiltration, ultrafiltration membranes. Indeed, among emerging products one can name nanofiber filters, carbon nanotubes and various nanoparticles.[9] Nanotechnology is expected to deal more efficiently with contaminants which convectional water treatment systems struggle to treat, including bacteria, viruses and heavy metals. This efficiency generally stems from the very high specific surface area of nanomaterials which increases dissolution, reactivity and sorption of contaminants.[10][11]

Some potential applications include:
  • To maintain public health, pathogens in water need to be identified rapidly and reliably. Unfortunately, traditional laboratory culture tests take days to complete. Faster methods involving enzymes, immunological or genetic tests are under development.[7]
  • Water filtration may be improved with the use of nanofiber membranes and the use of nanobiocides, which appear promisingly effective.[12]
  • Biofilms are mats of bacteria wrapped in natural polymers. These can be difficult to treat with antimicrobials or other chemicals. They can be cleaned up mechanically, but at the cost of substantial down-time and labour. Work is in progress to develop enzyme treatments that may be able to break down such biofilms.[7]

Pollution

Scientists have been researching the capabilities of buckminsterfullerene in controlling pollution, as it may be able to control certain chemical reactions. Buckminsterfullerene has been demonstrated as having the ability of inducing the protection of reactive oxygen species and causing lipid peroxidation. This material may allow for hydrogen fuel to be more accessible to consumers.
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Marsupial


From Wikipedia, the free encyclopedia


Marsupials are an infraclass of mammals living primarily in Australasia and the Americas. A distinctive characteristic, common to most species, is that the young are carried in a pouch. Well-known marsupials include kangaroos, wallabies, the koala, possums, opossums, wombats and the Tasmanian devil. Other marsupials include the numbat, bandicoots, bettongs, the bilby, quolls, and the quokka.

Marsupials represent the clade originating with the last common ancestor of extant metatherians. Like other mammals in the Metatheria, they are characterized by giving birth to relatively undeveloped young, often residing in a pouch with the mother for a certain time after birth. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with 13 in Central America, and one in North America north of Mexico.

Evolution

Isolated petrosals of Djarthia murgonensis, Australia's oldest marsupial fossils[3]

Dentition of the herbivorous eastern grey kangaroo, as illustrated in Knight's Sketches in Natural History

The relationships between the three extant divisions of mammals (monotremes, marsupials, and placental mammals) was long a matter of debate among taxonomists.[4] Most morphological evidence comparing traits such as number and arrangement of teeth and structure of the reproductive and waste elimination systems as well as most genetic and molecular evidence favors a closer evolutionary relationship between the marsupials and placental mammals than either has with the monotremes.[5]

Phylogenetic tree of marsupials derived from retroposon data[6]

The ancestors of marsupials, part of a larger group called metatherians, probably split from those of placental mammals (eutherians) during the mid-Jurassic period, though no fossil evidence of metatherians themselves are known from this time.[7] Fossil metatherians are distinguished from eutherians by the form of their teeth; metatherians possess four pairs of molar teeth in each jaw, whereas eutherian mammals (including true placentals) never have more than three pairs.[8] Using this criterion, the earliest known metatherian is Sinodelphys szalayi, which lived in China around 125 million years ago (mya).[9] This makes it a contemporary to some early eutherian species which have been found in the same area.[10]

The oldest metatherian fossils are found in present-day China.[11] About 100 mya, the supercontinent Pangaea was in the process of splitting into the northern continent Laurasia and the southern continent Gondwana, with what would become China and Australia already separated by the Tethys Ocean. From there, metatherians spread westward into modern North America (still attached to Eurasia), where the earliest true marsupials are found. Marsupials are difficult to distinguish from other fossils, as they are characterized by aspects of the reproductive system which do not normally fossilize (including pouches) and by subtle changes in the bone and tooth structure that show a metatherian is part of the marsupial crown group (the most exclusive group that contains all living marsupials). The earliest definite marsupial fossil belongs to the species Peradectes minor, from the Paleocene of Montana, dated to about 65 million years ago.[12] From their point of origin in Laurasia, marsupials spread to South America, which was connected to North America until around 65 mya. Laurasian marsupials eventually died off, possibly due to competition from placental mammals for their ecological niches.

In South America, the opossums evolved and developed a strong presence, and the Paleogene also saw the evolution of shrew opossums (Paucituberculata) alongside non-marsupial metatherian predators such as the borhyaenids and the saber-toothed Thylacosmilus. South American niches for mammalian carnivores were dominated by these marsupial and sparassodont metatherians. While placental predators were absent, the metatherians did have to contend with avian (terror bird) and terrestrial crocodylomorph competition. South America and Antarctica remained connected until 35 mya, as shown by the unique fossils found there. North and South America were disconnected until about three million years ago, when the Isthmus of Panama formed. This led to the Great American Interchange. Competition from placental mammals from the north drove sparassodonts to extinction, while didelphimorphs (opossums) invaded Central America, with the Virginia opossum reaching as far north as Canada.

Marsupials reached Australia via Antarctica about 50 mya, shortly after Australia had split off. This suggests a single dispersion event of just one species, most likely a relative to South America's monito del monte (a microbiothere, the only New World australidelphian). This progenitor may have rafted across the widening, but still narrow, gap between Australia and Antarctica. In Australia, they radiated into the wide variety seen today. Modern marsupials appear to have reached the islands of Borneo and Sulawesi relatively recently via Australia.[13][14][15] A 2010 analysis of retrotransposon insertion sites in the nuclear DNA of a variety of marsupials has confirmed all living marsupials have South American ancestors. The branching sequence of marsupial orders indicated by the study puts Didelphimorphia in the most basal position, followed by Paucituberculata, then Microbiotheria, and ending with the radiation of Australian marsupials. This indicates that Australidelphia arose in South America, and reached Australia after Microbiotheria split off.[16][17]

In Australia, terrestrial placental mammals disappeared early in the Cenozoic (their most recent known fossils being 55 million-year-old teeth resembling those of condylarths) for reasons that are not clear, allowing marsupials to dominate the Australian ecosystem.[13] Extant native Australian terrestrial placental mammals (such as hopping mice) are relatively recent immigrants, arriving via island hopping from Southeast Asia.[14]

Genetic analysis suggests a divergence date between the marsupials and the placentals at 160 million years ago.[18]
The ancestral number of chromosomes has been estimated to be 2n = 14.

A new hypothesis suggests that South American microbiotheres resulted from a back-dispersal from eastern Gondwana due to new cranial and post-cranial marsupial fossils from the Djarthia murgonensis from the early Eocene Tingamarra Local Fauna in Australia that indicate the Djarthia murgonensis is the most plesiomorphic, the oldest unequivocal australidelphian, and may be the ancestral morphotype of the Australian marsupial radiation. [19]

Description


Koala
(Phascolarctos cinereus)

Early development

An early birth removes a developing marsupial from its mother's body much sooner than in placental mammals, thus marsupials have not developed a complex placenta to protect the embryo from its mother's immune system. Though early birth puts the tiny newborn marsupial at a greater environmental risk, it significantly reduces the dangers associated with long pregnancies, as there is no need to carry a large fetus to full-term in bad seasons. Marsupials are extremely altricial animals, needing to be intensely cared for immediately following birth (cf. precocial).

Because newborn marsupials must climb up to their mother's nipples, their front limbs are much more developed than the rest of their bodies at the time of birth. This requirement possibly has resulted in the limited range of locomotor adaptations in marsupials compared to placentals. Marsupials must develop grasping forepaws during their early youth, making the transition from these limbs into hooves, wings, or flippers, as some groups of placental mammals have done, far more difficult.

An infant marsupial is known as a joey. Marsupials have a very short gestation period (about four to five weeks), and the joey is born in an essentially fetal state. The blind, furless, miniature newborn, the size of a jelly bean,[citation needed] crawls across its mother's fur to make its way into the pouch, where it latches onto a teat for food. It will not re-emerge for several months, during which time it develops fully. After this period, the joey begins to spend increasing lengths of time out of the pouch, feeding and learning survival skills. However, it returns to the pouch to sleep, and if danger threatens, it will seek refuge in its mother's pouch for safety.

Joeys stay in the pouch for up to a year in some species, or until the next joey is born. A marsupial joey is unable to regulate its own body temperature and relies upon an external heat source. Until the joey is well-furred and old enough to leave the pouch, a pouch temperature of 30–32 °C (86–90 °F) must be constantly maintained.

Reproductive system


Marsupials' reproductive systems differ markedly from those of placental mammals.[20][21] The female develops a kind of yolk sac in her womb which delivers nutrients to the embryo. Embryos of some marsupials additionally form placenta-like organs that connect them to the uterine wall, although it is not certain that they transfer nutrients from the mother to the embryo.[22] Pregnancy is very short, typically 4 to 5 weeks, and the embryo is born at a very young stage of development.[citation needed]

The evolution of reproduction in marsupials, and speculation about the ancestral state of mammalian reproduction, have engaged discussion since the end of the 19th century. Both sexes possess a cloaca,[21] which is connected to a urogenital sac used to store waste before expulsion. The bladder of marsupials functions as a site to concentrate urine and empties into the common urogenital sinus in both females and males.[21]

Male

Most male marsupials, except for macropods,[23] have a bifurcated penis, separated into two columns, so that the penis has two ends corresponding to the females' two vaginas.[24][25][21][26][27][28][29][30][31]The penis is used only for inseminating females, and is separate from the urinary tract.[32][21] The penis curves forward when erect,[33] and when not erect, it is retracted into the body in an S-shaped curve.[27] Neither marsupials nor monotremes possess a baculum.[26] The shape of the glans penis varies among marsupial species.[27][34][35][36] A male koala's foreskin contains naturally occurring bacteria that play an important role in fertilization.[37]

The male thylacine had a pouch that acted as a protective sheath, covering his external reproductive organs while he ran through thick brush.[38]

The shape of the urethral grooves of the males' genitalia is used to distinguish between Monodelphis brevicaudata, Monodelphis domestica, and Monodelphis americana. The grooves form 2 separate channels that form the ventral and dorsal folds of the erectile tissue.[39]

During the breeding season, the male tammar wallaby's prostate and bulbourethral gland enlarge. However, there does not appear to be any seasonal difference in the weight of the testes.[40]

Female

Female reproductive anatomy of several marsupial species

Female eastern grey kangaroo with a joey in her pouch

Female marsupials have two lateral vaginas, which lead to separate uteri, but both open externally through the same orifice. A third canal, the median vagina, is used for birth. This canal can be transitory or permanent.[26] The definitive placenta in all marsupials is generated by the yolk sac.[41] Among three fetal membranes in mammals, the yolk sac, allantois, and amnion, only the first two form a placenta.[42] The evolution of placentation in vertebrates is linked to the evolution of viviparity, a reproductive system in which the females retain their eggs to give birth to their young. Marsupials give birth at a very early stage of development (about four to five weeks); after birth, newborn marsupials crawl up the bodies of their mothers and attach themselves to a nipple, which is located on the underside of the mother either inside a pouch called the marsupium or open to the environment. To crawl to the nipple and attach to it, the marsupial must have well-developed forelimbs and facial structures.[43][44] This is accomplished by accelerating forelimb and facial development in marsupials compared to placental mammals, which results in decelerated development of such structures as the hindlimb and brain. There they remain for a number of weeks, attached to the nipple. The offspring are eventually able to leave the marsupium for short periods, returning to it for warmth, protection, and nourishment.

Characteristics

Marsupials are characterized by giving birth to relatively undeveloped young. They lack a complex placenta to protect the embryo from its mother's immune system. They have a front pouch containing multiple nipples for protection and sustenance of the young.

Some common structural features can be found among marsupials. Ossified patellae are absent in most modern marsupials, though a small number of exceptions are reported.[citation needed] Epipubic bones are present. Marsupials (and also monotremes) also lack a gross communication (corpus callosum) between the right and left brain hemispheres.[26]

Taxonomy


Sugar glider (Petaurus breviceps)

Common brushtail possum (Trichosurus vulpecula)

Squirrel glider
(Petaurus norfolcensis)

Virginia opossum (Didelphis virginiana), a North American marsupial

Thylacine (Thylacinus cynocephalus), an extinct carnivorous marsupial found in Tasmania until the 1930s

Taxonomically, the two primary divisions of Marsupialia are: American marsupials and the Australian marsupials.[1][2] The order Microbiotheria (which has only one species, the monito del monte) is found in South America, but is believed to be more closely related to the Australian marsupials. There are many small arboreal species in each group. The term 'opossums' is properly used to refer to the American species (though 'possum' is a common diminutive), while similar Australian species are properly called 'possums'.
† indicates extinction

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