Re: Howcome cells can split asexually and we(humans) cannot?

Hi Ralph,
That's a really good question. First off, I will say that humans have 
cells that do asexual division.  But since we are complex multicellular 
organisms we need both types of cell division.

I assume the cells you are taking about that split using asexually 
division are single cell organisms like yeast or bacteria.  Since these 
are single cells there is nothing else to divide to make more copies of 
themselves. In this case Reproduction (or sexual division) for them is 
the same as asexual division. They are simple enough that they don't ever 
change from what they are. A cell is a cell is a cell.

Asexual division is the same as mitosis for most of our cells. For 
instance, once we have a liver cell we only need asexual division to make 
more liver cells. Bacteria make identical bacteria, our liver cells make 
identical liver cells, both using asexual division

Sexual division, on the other hand, is used for offspring. Sexual 
division is also called meiosis.  The cells in our bodies, called 
gametes, divide sexually to produce the sperm and egg. When the sperm and 
egg come together, mitosis (asexual division) occurs.  This is how we 
humans can do both.

For humans to undergo only asexual division would be like spliting us in 
half and having each half capable of making the other. It would also make 
us all look exactly alike, which means that whatever the first human 
looked like, all other humans would look exactly identical. 

Both these types of divisions occur at the cellular level, not the 
organism level. Since bacteria or yeast cells are just one cell, their 
division is like having an offspring.  We humans must make gametes that 
when combined form into new offspring. We are much more complex and 
therefore must use different methods of reproducing than bacteria do.

One reason for this is in our cellular differentiation.  All cells start 
out the same, but at a certain time during development they change. Some 
will become your brain, others your stomach, and so on.  Once this 
occurs, they can only produce cells like themselves. Liver cells make 
liver cells only, they cannot make bone cells! Therefore to make copies 
of ourselves, we need another method

You will find that all organisms that undergo some sort of developmental 
process will have both asexual and sexual reproduction.

I hope this answers your question and never stop questioning or learning!

Matt