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Today, The Visible Embryo is linked to over 600 educational institutions and is viewed by more than 1 million visitors each month. The field of early embryology has grown to include the identification of the stem cell as not only critical to organogenesis in the embryo, but equally critical to organ function and repair in the adult human. The identification and understanding of genetic malfunction, inflammatory responses, and the progression in chronic disease, begins with a grounding in primary cellular and systemic functions manifested in the study of the early embryo.

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Pregnancy Timeline by SemestersFetal liver is producing blood cellsHead may position into pelvisBrain convolutions beginFull TermWhite fat begins to be madeWhite fat begins to be madeHead may position into pelvisImmune system beginningImmune system beginningPeriod of rapid brain growthBrain convolutions beginLungs begin to produce surfactantSensory brain waves begin to activateSensory brain waves begin to activateInner Ear Bones HardenBone marrow starts making blood cellsBone marrow starts making blood cellsBrown fat surrounds lymphatic systemFetal sexual organs visibleFinger and toe prints appearFinger and toe prints appearHeartbeat can be detectedHeartbeat can be detectedBasic Brain Structure in PlaceThe Appearance of SomitesFirst Detectable Brain WavesA Four Chambered HeartBeginning Cerebral HemispheresFemale Reproductive SystemEnd of Embryonic PeriodEnd of Embryonic PeriodFirst Thin Layer of Skin AppearsThird TrimesterSecond TrimesterFirst TrimesterFertilizationDevelopmental Timeline
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Home | Pregnancy Timeline | News Alerts |News Archive Dec 11, 2013

 

Desert tortoise (Gopherus agassizii) is unusual in that its mortality decreases as it gets older.

Image: Barbara Carroll




The freshwater medusa (Hydra magnipapillata) can live for centuries, with no decline in fertility.

Image: Ralf Schaible






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Aging and fertility out of bounds

Despite it being one the hottest topic in the media recently, scientists have no coherent explanation for aging. New demographic data on humans, animals and plants for the first time unveils an extraordinary diversity of aging processes that no existing evolutionary theory can explain.


Both life spans and mortalities vary from species to species. The fact that the probability of dying rises with age applies to humans, but is not true for all species.


A catalogue of 46 species with their respective mortality and fertility rates, published in the journal Nature, is the result of a long-term data collection project led by scientists at the Max Planck Institute for Demographic Research (MPIDR) in Rostock, Germany, and at the Max-Planck Odense Center on the Biodemography of Aging (MaxO) in Odense, Denmark.

Not only are previous explanations unable to deal with life spans ranging from a few days (fruit fly), to decades (humans), to centuries (hydra), but they are also unable to account for variations in the death rate.


Common theories assert that the probability of dying rises with age, as it does for humans. However, the researchers cataloged species such as the white mangrove and the desert tortoise whose probability of dying actually decreases with age. In addition, fertility periods of some species also challenge common theories.


Previous attempts to explain aging claim that creatures only invest in self-preservation until they have reproduced successfully and raised their offspring. Following this line of reasoning, when the end of the fertility period approaches, the body should start to decay – which is known as senescence, or aging.

For humans this is only partly true. According to the Nature study, mortality of modern Japanese women rises constantly after childhood. But contradictorily, humans still live for a long time after fertility has ceased. Today, many people stay healthy until they are grandparents and their probability of dying is correspondingly small. Only at advanced ages is mortality growing rapidly. For example, in Japanese women 100 years old, mortality reaches more than 20 times their lifetime average.

This makes humans a real oddity. No other species in the researcher’s catalog has a mortality curve which rises that sharply. Even among other mammals, death rates reach no more than five times the lifetime average. Why evolution developed such big differences is a mystery to Biologists.

For many species aging is turned upside down

Current theories are especially at odds with two groups of species for which the concept of aging appears to be turned upside down. There are creatures whose mortality stays constant throughout their whole life, like hydra or the hermit crab. Their bodies do not seem to degenerate during their lifetime which can be understood as the absence of aging. And there are even species whose probability of dying decreases as they grow older, like the red gorgonian (a coral), the netleaf oak and the desert tortoise. Their risk of dying obviously never becomes zero, but when they are old they are more likely to survive until their next birthday than when they were in their youth.


There is another belief that the new data catalogue disproves: the idea that species with a short life span die so soon because they age so quickly.

This would mean that their mortality rises strongly throughout life. However, sometimes the contrary is the case, such as in the tundra vole. Its mortality increases only moderately until it reaches two times its lifetime average at old age. Nevertheless, this vole rarely survives beyond one year.

Humans, however, are living for an entire century more and more often, despite the fact that their risk of dying skyrockets in old age (up to more than 20 times the lifetime average).


Data will pave the way for a unified theory of aging

“Surprisingly, one can hardly imagine a type of life course that is not found in nature,” says MaxO researcher Owen Jones. This applies not only for mortality but also for fertility.

While women become infertile after a limited childbearing period in the first half of their lives, fertility rises until almost the end of the lifespan for a bird - the alpine swift. And the yellow baboon has offspring throughout its life without any influence of age.

“One reason why we still lack a unified theory of aging is that our view is based on data for a very restricted selection of species,”
says biodemographer Alexander Scheuerlein from MPIDR.

There has long been high quality demographics for hundreds of mammals and birds, but very few for other vertebrates or invertebrates. Extremely little is known about algae, fungi or bacteria. In order to understand why evolution created aging, more data on all species has to be collected, says Alexander Scheuerlein.

Abstract
Evolution drives, and is driven by, demography. A genotype moulds its phenotype’s age patterns of mortality and fertility in an environment; these two patterns in turn determine the genotype’s fitness in that environment. Hence, to understand the evolution of ageing, age patterns of mortality and reproduction need to be compared for species across the tree of life. However, few studies have done so and only for a limited range of taxa. Here we contrast standardized patterns over age for 11 mammals, 12 other vertebrates, 10 invertebrates, 12 vascular plants and a green alga. Although it has been predicted that evolution should inevitably lead to increasing mortality and declining fertility with age after maturity, there is great variation among these species, including increasing, constant, decreasing, humped and bowed trajectories for both long- and short-lived species. This diversity challenges theoreticians to develop broader perspectives on the evolution of ageing and empiricists to study the demography of more species.

This article orignially appeared at the Max Planck-Gesellschaft Institute along with related articles of interest on humans who are over 110 years old: http://www.mpg.de/7646867/diversity-aging