Human Eye

Human eye is a very complex structure. It can distinguish about 10 million colors. There are rod and cone cells in the retina which allow conscious light perception and vision including color differentiation and the perception of depth.

According to the evolution theory of Darwin, the living things evolved from simple to the  the complex through gradations. So evolution of the eye poses a challenge to the Darwin’s theory because eye consists of many parts intricately connected to each other. The critics argue how could something so complex, they argue, have developed through random mutations and natural selection, even over millions of years?

If evolution occurs through gradations, the critics say, how could it have created the separate parts of the eye — the lens, the retina, the pupil, and so forth — since none of these structures by themselves would make vision possible?

Darwin acknowledged from the start that the eye would be a difficult case for his new theory to explain. Difficult, but not impossible. Scientists have come up with scenarios through which the first eye-like structure, a light-sensitive pigmented spot on the skin, could have gone through changes and complexities to form the human eye, with its many parts and astounding abilities.

Through natural selection, different types of eyes have emerged in evolutionary history — and the human eye isn’t even the best one, from some standpoints. Because blood vessels run across the surface of the retina instead of beneath it, it’s easy for the vessels to proliferate or leak and impair vision. So, the evolution theorists say, the anti-evolution argument that life was created by an “intelligent designer” doesn’t hold water: If God or some other omnipotent force was responsible for the human eye, it was something of a botched design.

Biologists use the range of less complex light sensitive structures that exist in living species today to hypothesize the various evolutionary stages eyes may have gone through.

Here’s how some scientists think some eyes may have evolved: The simple light-sensitive spot on the skin of some ancestral creature gave it some tiny survival advantage, perhaps allowing it to evade a predator. Random changes then created a depression in the light-sensitive patch, a deepening pit that made “vision” a little sharper. At the same time, the pit’s opening gradually narrowed, so light entered through a small aperture, like a pinhole camera.

Every change had to confer a survival advantage, no matter how slight. Eventually, the light-sensitive spot evolved into a retina, the layer of cells and pigment at the back of the human eye. Over time a lens formed at the front of the eye. It could have arisen as a double-layered transparent tissue containing increasing amounts of liquid that gave it the convex curvature of the human eye.

In fact, eyes corresponding to every stage in this sequence have been found in existing living species. The existence of this range of less complex light-sensitive structures supports scientists’ hypotheses about how complex eyes like ours could evolve. The first animals with anything resembling an eye lived about 550 million years ago. And, according to one scientist’s calculations, only 364,000 years would have been needed for a camera-like eye to evolve from a light-sensitive patch.


Eyelashes: Protector of Eyes

Every organ or part of our bodies is designed to perform some biological function for the survival and fitness of our bodies. Some of the organs keep working without our ever noticing it until that organ stops working due to some injury or illness. Only then we realize the importance of that organ. One such part of body is eyelashes.

For centuries the eyes have been recognized as an important part of physical beauty, especially for women. Eyes can communicate without the need of words. The human beings have developed great capabilities of eyes. Attractive women’s eyes are often also associated with favorable social status. And in many cultures long full eye lashes are symbols of beauty. On the other hand, loss of eye lashes is seen as a sign of deficiency in women. And so for centuries women have tried a myriad of methods for making their lashes, longer and fuller.

These beauty part aside, from an anatomy and physiology point of view, however, eye lashes serve several functions. They are intended to keep foreign particles or small insects from entering the eyes and causing damage or irritation. Lashes are attached to eyelids in a curved arc designed to channel water away from the eyes, forming another layer of protection from the environment. Lashes are actually sensitive structures, similar to cats’ whiskers. They trigger the blink reflex response when an object comes too close to the eyes

Lashes are simply hairs that grow from the edge of the eyelid. They are arranged in two or three rows. Each eye has between 100 and 150 individual hairs with upper lids having the greater number. Eyelashes are the widest type of human hair and the most richly pigmented. Each hair is, on average, 8-9 mm long, 7 mm of which extend beyond the eyelid. Lashes grow at a rate of about 0.15 mm per day, which means that if lashes are pulled out they take about eight weeks to fully grow back. Like other human hair, eyelashes are produced from follicles under the skin. Follicles have three stages of growth—an actual growing phase, a declination phase and a shedding phase. Each hair is very strong—capable of supporting 100 grams.

Hair growth in humans is different from hair growth in many mammals that shed their hair all at once. Human hair growth is asynchronous—that is, some follicles are experiencing growth while others are in decline or being shed. The eyelash growth cycle is variable, lasting between five and twelve months. The first phase is called anagen. This is the growing phase and lasts about 45 days on average. The normal length of a person’s lashes is determined by this phase. In the second stage of growth, called catagen, follicle cells undergo programmed cell death, a process that takes about fifteen days. About 3% of all lashes are in this phase at any given time. The lashes then enter a period of rest, telogen, which can last as long as nine months. Up to 15% of hair is in this phase. At the end of this phase lashes are shed in a process labeled exogen. As this phase ends, anagen begins again.

Like all human hair, eyelashes are made of 85% proteins, primarily keratin and melanin. Water makes up 7 % of human hair and 3 percent is composed of lipids. The keratin is made up of eighteen amino acids, 7 percent water, and low concentrations of trace minerals (e.g., iron, zinc, copper).