During fertilization in multicellular organisms, a sperm cell fuses with an egg cell to form a zygote. then, the zygote begins to divide. first, one cell becomes two, then two cells become four, four cells become eight and so on and so forth until a large ball of identical cells is formed. at this point, the ball begins to change. a space forms in the middle, and the cells are pushed inward to form layers. if cell differentiation begins after this phase, what most likely triggers this process?

relative dating involves things like higher layers are most recent than rocks lower in the sequence. this can be extended to the known sequence of fossils. rocks containing rabbit fossils are later than those containing dinosaur fossils.  

absolute dating methods mainly use radioactive elements that have known rates of decay and produce recognizable products. for example potassium 40 decays to produce argon. once a rock has solidified the argon is trapped. by measuring the argon and potassium0 you can get an accurate number for the age.  

some other methods are good for archaeological ages. dendrochronology involves recognizing the sequence of tree rings. the orientation of magnetized particles in baked clay can be used to date a fire.

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