Much talk has revolved around coming pole shifts on Earth, either physical or magnetic. Does this evidence bear out such an event happening ?
An 180-degree physical inversion of the planets's solid crust around the molten core would be the ultimate cataclysmic "Earth change". However, more than a few experts find this scenario most likely only if another planetary body...such as a "rogue planet" or similar cosmic anomaly...passed in close proximity to the Earth, providing enough torque for such an event to occur.
A reversal of the magnetic poles is much more likely, however, as it's happened before. Seen in the paleomagnetic record, locked into the rocks of the ocean floor and in some lava flows, scientists think the time between reversals on the Earth, falls somewhere between every 100,000 to 25 million years, with the reversal itself taking about 5,000 years. 
Not that just the reversal takes that long, mind you, and therein lies the crux of the problem. No...during much of that time, the geomagnetic field strength decreases...which counts as part of the reversal process. According to laboratory analysis of thermorenanent magnetization carried by clay baked in ancient times, the average global field intensity in any given region went through a broad maximum about 2,000 years ago.  After reaching a maximum, it then began its long descent.
NASA's Magnetic Field Satellite concurs, confirming a trend that goes back to at least 1830, namely, that the terrestrial magnetic field is decreasing in strength. At a rate measured by the satellite, the Earth's field will hit "zero" in about 1,200 years.  That, at least, is the estimate.
One contributing factor to the decrease, and eventual reversal, may be intensifying core spots within the inner core that are magnetized in a sence opposite of the main field ...while competing energy sources, such as heat loss at the mantle-core boundary (itself possibly contributing to climate change) and growth of the inner core may also be contributing factors. 
As the field reaches "zero", or close to it (some estimate the magnetic field will be at 20% of normal during a reversal), complete with periods of rapid and radical changes , something much more threatening than the upcoming reversal will occur.
First, as the field weakens, so does what many believe is the natural navigatory force for many living creatures. Studies have shown that whales, dolphins, birds, and even mollusks and mice use the geomagnetic field as a guiding device. Some, like mice, use it as a directional guide ...while cetaceans use the total geomagnetic field as a map, not for directional informationn, as we would use a compass, but by navigating the contours of magnetic "hills and valleys" in a field that is anything but uniform. 
Without it, whales and dolphins would beach in record numbers...birds and other creatures would be lost...unable to find their homes.
The effect of a magnetic reversal on humans is less known. Confusion might play a part, and some theorize a complete "reboot" of the human mind...the mind IS known to contain magnetite...might occur. However, as the field decreases (before a reversal), one could surmise what may happen by looking at a recent experiment by Dr. Valerie Hunt. Hunt had a room constructed in which the magnetic field intensity could be varied. The following...as spelled out in her book, "Infinite Mind", was observed:
When..."the magnetism was decreased, gross incoordination occurred. The entire neurological integrating mechanism was thrown off. Subjects could not balance their bodies and they had difficulty touching finger to nose or performing simple coordinated movements. They lost kinesthetic awareness". 
Could our bodies adjust as a much more gradual, long-term but worldwide decrease in the magnetic field intensity occurred ? That may be doubtful, and if so, then at what point near the conclusion of this pre-reversal descent would such incoordination sweep over the human race ?
Not that it would matter for long. Now, as the Earth and its magnetosphere plows through the solar wind, the charged particles are diverted around a "shell" created by the goemagnetic field. Without that field, high-energy particles...including dangerous gamma and X-rays, would penetrate completely, to the surface. If television or radio were still in use at that point in the future, transmissions would become impossible. Electric power grids would be off-line indefinitely.
And the effects on organic matter ??
Gamma rays, because they are so penetrating, can have severe effects on the cells of humans and other animals. As with cosmic rays, which also bombard the Eath at all times...gamma rays are known as ionizing radiation, which can cause a host of problems form cell death to genetic mutations (leading to cancer), in anny living thing.  As was said before, the magnetic pole reversal itself would be the least of the worries...as it's unlikely future generations would survive long enough to see it.
 "Geomagnetic Intensity as Evaluated from Ancient Pottery", Nature, 328:330, 1987
 "Magsat Down: Magnetic Field Declining", Science News, 117:407, 1980
 "Inner Earth Coming Out", Science News, 131:222, 1987
 "What Triggers Reversals of the Earth's Magnetic Field?", Nature, 309:115, 1984
 "How the Geomagnnetic Field Vector Reverses Polarity", Nature, 316:230, 1985
 "Magnetic Orientation on Small Mammals", Journal of Mammalogy, 70:1, 1989
 "No Through Road for the Misguided Whale", New Scientist, p.46, Feb. 12, 1987
 "Infinite Mind", Dr. Valerie Hunt, pp.31
 "How Nuclear Radiation Works", BYG Publishing, 1998