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At the cellular level, we do not have much difference with a spider or a fly: the case of Wnt

A prediction of the theory of evolution is the most basic and fundamental aspects of living beings should be well preserved, though very different animals compare with each other (these features should appear very early in the course of evolution.) On the other hand, the more "macro" and general (eg system level and physiology), may eventually be quite different from animals living in different (and everyone should have adapted to different conditions, being selected generation after generation, natural selection).
This is a prediction of the theory, and makes sense, but needs to be verified experimentally (as any scientific theory). The good news is that it has been tested systematically for decades, consider a particular case:

compare to a fruit fly (Drosophila melanogaster ) with you, dear reader / a (Homo sapiens ):

• You are a chordate, part of the class of mammals. On the other hand, the fly is an arthropod, which belongs to the class of insects.
• You have a skeleton as such (subphylum of vertebrates). On the other hand, we do not have it fly, but has an enviable armor.
• You have a closed circulatory system with a heart tetraventricular complex and real blood (no need to cut your finger to check). On the other hand, the fly has an open circulatory system, where the "blood directly bathes the tissues. But this "blood" is actually called "hemolymph" and not for anything red.
• You have a respiratory system with 2 powerful lungs, in charge of distributing the oxygen through tissues via the blood. On the other hand, flies do not have a respiratory system as such, rather have a series of "pipes" that run through your body, called tracheae, through which air enters from the outside by diffusion, giving oxygen to tissues without a liquid transporting.

If we continue with the comparison at this level, we find differences appalling and think that insects are aliens, totally alien to our nature. But let's split hairs:

• You are a metazoan eukaryotic cell (with a true nucleus), the fly too!
• Your cells use a way to communicate things from the outside in, called Wnt (pronounced "wint"), flies too!, But not 100% identical version.

This image shows the main "transduction" of cells:

The Wnt pathway is indicated by the arrow. We'll get into the detail of the cell biology of the case (not important for the development of the idea.) Just understand that "Wnt" is a key that comes to lock, which is located in the cell membrane. Once that occurred
interaction triggers a series of steps (one after another) within the cell. What is intended to reach the nucleus and regulate the expression of some genes.
If we compare this route specified between humans and flies, we found a similar creepy! are the same proteins in the same provision, with only slight differences (so called "peer review"). In addition, several homologues of each protein in each animal on its own (different versions), but to simplify our model, we think it is only one for each step of this series.
In humans, the foreign key is called "Wnt1" flies, known as " wingless (wg ), which is a gene" polarity segments ", whose job is to generate patterns in the embryo , as shown in the image below:

cells expressing wingless in this scheme are black circles. This gene is active in places where a group of genes, known as genes "Rule of the couple" are not expressed specifically even-skipped (eve ) and fushi tarazu (ftz ). This creates a pattern of segments and subdivisions of these, called parasegment, each with an anterior and posterior compartment.
neighboring cells wingless cells express a gene called engrailed ( in ). This is active, unlike wingless, where genes "pair rule" are active. Thus, engrailed is present on the trailing edge of each segment, or in the anterior compartment of each parasegment (in another try to understand trial weapon as a fly).
In short, cells wingless and engrailed cells conversing with each other during this part of the development on an ongoing basis to generate this pattern parasegmentario the fly embryo.
In humans, one of the homologues of wingless called "Wnt1" which is a proto-oncogene (genes that promote growth and cell division, but if something is wrong with them, can trigger uncontrolled growth, causing cancer). This protein is very similar to wingless and both are exactly the same route within the cell, only participants have different names. Wnt1
also has an important role in the development of all vertebrates. Therefore, in addition to corroborating the prediction of the conservation of pathways at the cellular level, we speculate that the Wnt pathway is very old and already possessed the animal that gave rise to both arthropods and chordates. That explains why in today spiders, crabs, ants, flies, elephants, giraffes, dogs, cats and humans, we use basically the same tool for different applications.