Scientists combine several well-tested techniques to find out the ages of fossils. The most important are Relative Dating, in which fossils and layers of rock are placed in order from older to younger, and Radiometric Dating, which allows the actual ages of certain types of rock to be calculated.
Relative Dating. Fossils are found in sedimentary rocks that formed when eroded sediments piled up in low-lying places such as river flood plains, lake bottoms or ocean floors. Sedimentary rock typically is layered, with the layers derived from different periods of sediment accumulation. Almost any place where the forces of erosion - or road crews - have carved through sedimentary rock is a good place to look for rock layers stacked up in the exposed rock face. When you look at a layer cake, you know that the layer at the bottom was the first one the baker put on the plate, and the upper ones were added later. In the same way, geologists figure out the relative ages of fossils and sedimentary rock layers; rock layers, and the fossils they contain, toward the bottom of a stack of sediments are older than those found higher in the stack.
Radiometric Dating. Until the middle of the last century, "older" or "younger" was the best scientists could do when assigning ages to fossils. There was no way to calculate an "absolute" age (in years) for any fossil or rock layer. But after scientists learned that the nuclear decay of radioactive elements takes place at a predictable rate, they realized that the traces of radioactive elements present in certain types of rock, such as hardened lava and tuff (formed from compacted volcanic ash), could be analyzed chemically to determine the ages, in years, of those rocks.
Putting Relative and Radiometric Dating Together. Once it was possible to measure the ages of volcanic layers in a stack of sedimentary rock, the entire sequence could be pinned to the absolute time scale. In the Wyoming landscape shown below left, for example, the gray ash layer was found to be 73 million years old. This means that fossils in rock layers below the tuff are older than 73 million years, and those above the tuff are younger. Fossils found embedded within the ash, including the fossil leaves shown below right, are the same age as the ash: 73 million years old. So, there for the relationship between the age of a fossil and the layer of rock in which it was found dates back to about the same time era.
- Got this out of an article, hope it helps! (:
Answer:
When a pathogens cross non-specific barriers (skin) they disturb the homeostasis of the body. however inside body they are confronted by different types of white blood cells which are responsible for bringing homeostasis again in the body. these are described below.
Role of macrophages in maintaining homeostasis
Macrophages are white blood cells (phagocytes) constantly patrolling in the body and destroy other cells by engulfing and ingesting them through the process of phagocytosis. These are produced in bone marrow cells as monocytes which have short life of only 10-20 hours. Once monocytes leave the blood they become macrophages. From bone marrow to the blood, macrophages are transported to the area of the body where they are needed. Macrophages engulf the whole body, after digesting particles (bacteria) can extrude the residual particles. Thus, provide protection by trapping and destroying microorganisms entering the tissue. They also secrete different proteins. Some of these proteins trigger the maturation of monocytes into macrophages, thereby increasing their numbers. Another protein interleukin-1 signals the brain to raise the body temperature, producing fever. The higher temperature aids the immune response and inhibit the growth of invading microorganisms.
Role of natural killer cells in maintaining homeostasis
They are another class of white blood cells that don’t directly attack invading microbes. Instead they strike at the body’s own cells that have been invaded by viruses. Virus infected proteins bear viral proteins on their surfaces. Natural killer cells recognize and kill cancerous cells. They secrete proteins into plasma membrane of the infected or cancerous cells. They also secrete enzymes that break up some of the molecules of the target cells, as a result the target cell soon dies.
A substance, either an ion or an electrically neutral molecule, formed by the union of simpler substances (as compounds or ions) and are held together by forces that are chemical rather than physical.
