Life Science

Hi!

A refresher: About four (4) years ago, this blog/page was created by me, then I entitled it-LIFE SCIENCE- and I've been able to record a few topics here when I was studying my Bachelor's degree. Having returned after graduation, I'm inspired to recreate this webpage by continuing to share all of my acquired knowledge while I still have time to learn and pursue a lot more interesting opportunities in the future.

However, looking back at a few previous topics, there appears to be a lack of more details. For this reason, I am prepared to gather all I can, including data facts, new ideas, relevant topics, and some research studies, in order to fulfill the needs and wants of other science learners in the world.

To be honest, I cannot tell any more drama, but I'm simply glad to be back and take enough time now than before. Though I've been busy developing some new skills today, I'll try my best to bring additional information on this page. Because it's my first blog, I probably wouldn't be able to provide regular updates; after all, learning takes time and practice, so please bear with me.

♒

While early hominins had important biological differences, they also shared a number of essential characteristics. Most of them were probably just about as efficient as humans were at bipedal movement (3 million years ago).

The bones of their pelvis, or hip area, were reduced from top to bottom and bowl-shaped, similar to humans but not apes. This increased the stability of the pelvis for weight - bearing activities when standing erect or moving bipedally.

Australopithecine and other early hominin fossils have been found only in Africa.  The majority of them were discovered in East and South Africa.  However, some also were found in Chad, which is located in North Central Africa.  Current evidence indicates that there were as many as 12 species of early hominins between 6 and 1.5 million years ago, but they did not all live at the same time.  The following species are the most widely accepted ones:

1. Australopithecus anamensis

2.Australopithecus afarensis

3.Australopithecus africanus

4.Paranthropus aethiopicus (or Australopithecus aethiopicus)

5.Paranthropus boisei (or Australopithecus boisei)

6.Paranthropus robustus (or Australopithecus robustus)

There has been a breach in the fossil hominin record for the critical time preceding the appearance of Australopithecus anamensis 4.2 million years ago. New findings are now emerging to fill in the gaps in the picture of evolution that led to the emergence of the australopithecines at that early stage.

Common and Trade Names of Chemicals  

Animal Sciences COPYRIGHT 2002 The Gale Group Inc.

Cnidaria is one of the more primitive animal phyla. It includes aquatic organisms such as jellyfish, sea anemones, corals, and hydras. Most cnidarians are marine, although a few, such as the well-known hydra, are freshwater species.

⚬Characteristics of Cnidarians

All cnidarians are characterized by radially symmetric body plans, rather than the bilaterally symmetric body plans that are found in most other animal phyla. Although cnidarians are more advanced than sponges (phylum Porifera) in that they possess distinct tissue layers, they lack many of the features of more advanced animal phyla, such as internal organs and central nervous systems. Most cnidarians possess tentacles, and many also have nematocysts (specialized stinging cells). Both are involved in feeding.

Cnidarians are characterized by the presence of three tissue layers, an outer protective epidermis, a middle layer called the mesoglea, and an inner layer called the gastrodermis, whose function is primarily digestive. The mesoglea of cnidarians is not as highly developed as the mesoderm of other animal groups, being primarily gelatinous with only a few fibrous or amoeba-like cells.

Cnidarians possess only one digestive opening, which serves as both the mouth and the anus. This opening is surrounded by tentacles and leads to an internal digestive cavity called the gastrovascular cavity .

Cnidarians feed using tentacles that are embedded with stinging nematocysts. The nematocysts are springing barbs with small hairlike triggers that are activated by contact with prey. Most nematocysts require stimulation in more than one sensory mode before they will fire. For example, a nematocyst may respond only if there is mechanical stimulation from physical contact with the prey as well as chemical stimulation signaling the presence of suitable prey. As nematocysts fire, barbs unfold and become embedded in the tissue of the prey. At the same time, the nematocysts inject the prey with an immobilizing toxin through a long hollow thread within the barb. Once the prey item has been captured and subdued, tentacles are used by the cnidarians to bring the prey item into the gastrovascular cavity. Within the gastrovascular cavity, the food item is broken into small particles by digestive enzymes secreted by gastrodermal cells lining the cavity. The minute particles are then taken in by the gastrodermal cells, and digestion is completed in digestive vacuoles (small cavities) within these cells. The indigestible remnants of the prey are expelled from the mouth of the gastrovascular cavity.

One hypothesis about the origin of nematocysts suggests that they were prokaryotic endosymbionts which lived within eukaryotic cells as mutualists (mutualisms are symbiotic relationships between individuals of two different species, in which members of both species derive benefits from the relationship), the same way organelles (specialized parts of cells) such as mitochondria and chloroplasts are believed to originate.

Unlike more advanced animal phyla, cnidarians lack a central nervous system. Instead, their nerves are organized in nerve nets that cover the entire body. Impulses spread slowly out from the point of stimulation along the nerve net. Some cnidarians, such as jellyfish, have more complicated arrangements of nerves that allow for more complex responses to stimuli as well as more effective patterns of movement.

Cnidarians also lack certain tissue types found in other animal phyla, such as true muscle cells. However, they do have fibers that can contract and therefore can be used in capturing prey and in moving about.

⚬Major Groups of Cnidarians

Cnidarians are divided into three major classes. These are the Hydrozoa (hydras and other colony-forming species), the Scyphozoa (jellyfish), and the Anthozoa (sea anemones and corals).

🔶Hydrozoa. The best-known member of the Hydrozoa is the hydra, a freshwater species. However, the hydra is not a typical hydrozoan. For example, the hydra has only a polyp stage, for example, whereas most hydrozoans have a biphasic (two-stage) life cycle that alternates between a sedentary polyp stage and a mobile, bell-shaped medusa stage. The hydra is not strictly sedentary; it moves in a very unusual way, by turning somersaults. In addition, most hydrozoans are colonial, with each colony arising from the asexual budding of a single individual. Members of a hydrozoan colony have interconnected gastrovascular cavities, and the fluid in this cavity is circulated by cells with long, beating flagella . There is typically some degree of division of labor within the colony. Usually, there are feeding polyps, which possess tentacles and nematocysts (stinging cells), and reproductive polyps, which continually bud off tiny mobile medusas. The medusas swim by tightening and relaxing cells within the bell, and are also scattered by prevailing water currents. Medusas release sperm and eggs directly into the water, where fertilization occurs. The zygote (fertilized egg) develops into what is called a planula larva—the larvae of cnidarians. The larva ultimately settles to the substrate (rocky bottom of the ocean), finds something to anchor to, develops a mouth and tentacles, and becomes a polyp that subsequently buds to form a new colony.

🔹Scyphozoa. The Scyphozoa includes the well-known jellyfish. In this group, the polyp stage is far less significant than among the Hydrozoa, since the medusa stage is dominant. Scyphozoan medusas grow to sizes considerably larger than those found among the Hydrozoa. They range in size from a few centimeters to over 2 meters across. The nervous systems of jellyfish are also more developed than those of other cnidarians. Instead of a simple nerve net, they have a nerve ring around the edge of the bell portion of the medusa. Neurons throughout the rest of the body connect to this ring. This organization allows for faster conduction of impulses from one side of the body to the other, which in turn allows the jellyfish to swim with coordinated contractions of the entire bell.

MISCONCEPTIONS OF EVOLUTION [one] The website Understanding Evolution, hosted by The University of California Museum of Paleontology, Berkeley, offers its readers numerous helpful resources regarding the science and history of evolutionary biology. The site’s stated goal is to “help you understand what evolution is, how it works, how it factors into your life, how research in evolutionary biology is performed, and how ideas in this area have changed over time.” Among its resources is a list of popular misconceptions about evolutionary theory. In this two part series, we’d like to highlight some of the site’s most helpful responses to these misconceptions. The full list, and many other wonderful resources, can be found at Understanding Evolution.

Misconceptions about Evolutionary Theory and Process

“Evolution is a theory about the origin of life.”

Evolutionary theory does encompass ideas and evidence regarding life’s origins (e.g., whether or not it happened near a deep-sea vent, which organic molecules came first, etc.), but this is not the central focus of evolutionary theory. Most of evolutionary biology deals with how life changed after its origin. Regardless of how life started, afterwards it branched and diversified, and most studies of evolution are focused on those processes.

For more, see our questions on “What is Evolution?” and “Isn’t the Origin of Life Highly Improbable?”.

“Evolution is like a climb up a ladder of progress; organisms are always getting better.”

One important mechanism of evolution, natural selection, does result in the evolution of improved abilities to survive and reproduce; however, this does not mean that evolution is progressive — for several reasons. First, natural selection does not produce organisms perfectly suited to their environments. It often allows the survival of individuals with a range of traits — individuals that are “good enough” to survive. Hence, evolutionary change is not always necessary for species to persist. Many taxa (like some mosses, fungi, sharks, opossums, and crayfish) have changed little physically over great expanses of time. Second, there are other mechanisms of evolution that don’t cause adaptive change. Mutation, migration and genetic drift may cause populations to evolve in ways that are actually harmful overall or make them less suitable for their environments. For example, the Afrikaner population of South Africa has an unusually high frequency of the gene responsible for Huntington’s disease because the gene version drifted to high frequency as the population grew from a small starting population. Finally, the whole idea of “progress” doesn’t make sense when it comes to evolution. Climates change, rivers shift course, new competitors invade — and an organism with traits that are beneficial in one situation may be poorly equipped for survival when the environment changes. And even if we focus on a single environment and habitat, the idea of how to measure “progress” is skewed by the perspective of the observer. From a plant’s perspective, the best measure of progress might be photosynthetic ability; from a spider’s it might be the efficiency of a venom delivery system; from a human’s, cognitive ability. It is tempting to see evolution as a grand progressive ladder with Homo sapiens emerging at the top. But evolution produces a tree, not a ladder — and we are just one of many twigs on the tree.

For more, see our questions “What is Evolution?” and “Did Evolution Have to Result in Human Beings?”.

“Evolution means that life changed ‘by chance.’”

Chance and randomness do factor into evolution and the history of life in many different ways; however, some important mechanisms of evolution are non-random and these make the overall process non-random. For example, consider the process of natural selection, which results in adaptations — features of organisms that appear to suit the environment in which the organisms live (e.g., the fit between a flower and its pollinator, the coordinated response of the immune system to pathogens, and the ability of bats to echolocate). Such amazing adaptations clearly did not come about “by chance.” They evolved via a combination of random and non-random processes. The process of mutation, which generates genetic variation, is random, but selection is non-random. Selection favored variants that were better able to survive and reproduce (e.g., to be pollinated, to fend off pathogens, or to navigate in the dark). Over many generations of random mutation and non-random selection, complex adaptations evolved. To say that evolution happens “by chance” ignores half of the picture.

For more see our questions on “What is Evolution?” and “How do randomness and chance align with belief in God’s sovereignty and purpose?”.

“Humans are not currently evolving”

Humans are now able to modify our environments with technology. We have invented medical treatments, agricultural practices, and economic structures that significantly alter the challenges to reproduction and survival faced by modern humans. So, for example, because we can now treat diabetes with insulin, the gene versions that contribute to juvenile diabetes are no longer strongly selected against in developed countries. Some have argued that such technological advances mean that we’ve opted out of the evolutionary game and set ourselves beyond the reach of natural selection — essentially, that we’ve stopped evolving. However, this is not the case. Humans still face challenges to survival and reproduction, just not the same ones that we did 20,000 years ago. The direction, but not the fact of our evolution has changed. For example, modern humans living in densely populated areas face greater risks of epidemic diseases than did our hunter-gatherer ancestors (who did not come into close contact with so many people on a daily basis) — and this situation favors the spread of gene versions that protect against these diseases.

“Species are distinct natural entities, with a clear definition, that can be easily recognized by anyone.”

Many of us are familiar with the biological species concept, which defines a species as a group of individuals that actually or potentially interbreed in nature. That definition of a species might seem cut and dried — and for many organisms (e.g., mammals), it works well — but in many other cases, this definition is difficult to apply. For example, many bacteria reproduce mainly asexually. How can the biological species concept be applied to them? Many plants and some animals form hybrids in nature, even if they largely mate within their own groups. Should groups that occasionally hybridize in selected areas be considered the same species or separate species? The concept of a species is a fuzzy one because humans invented the concept to help get a grasp on the diversity of the natural world. It is difficult to apply because the term species reflects our attempts to give discrete names to different parts of the tree of life — which is not discrete at all, but a continuous web of life, connected from its roots to its leaves.