We humans tend to over-estimate our differences with other animals. But if you look at the basic anatomy of other mammals, and even some of our more distant evolutionary cousins, you'll notice that we share many of the same underlying traits. We have a common set of biological tools for interacting with the world, sharing close to 90% of our genes with other mammals.
Our basic genetic formula was determined long ago. Almost two billion years of evolutionary experimentation was spent simply developing the basic structures of single cell organisms. But through the eons our genetic formula became exponentially more advanced, eventually allowing for multicellular life. It was within the oceans that our evolutionary ancestors first developed simple brains, central nervous systems, circulatory systems, sexual reproduction, & the skeletal system. And it was hundreds of millions of years ago that we evolved our senses as the mind's gateway to the external world. All the mammals that we share the Earth with inherited a mostly similar set of blueprints - similiar bones cells, blood cells, brain cells...
Regardless of the final form, all vertebrates still rely on much of this ancient genetic code to create the main components of our bodies. For hundreds of millions of years their (typically) four limbed skeletal system has proved itself effective at adapting to the planet's continuously changing demands. Every bone within our bodies has a correlate in the skeletons of other mammals. In the case of humans however, our number of tail vertebrate has significantly decreased to become the Coccyx (tailbone).
Today our study of genetics is showing us in detail how the evolutionary tree diverges. These genetic findings are confirming much of what Darwin could discern simply by observing the fossils & by making anatomical comparisons.
For a species to morph into a perceivably different form only requires a series of relatively minor genetic mutations, particularly within the genes controlling bone growth. The process that vertebrate's genetic code unfolds into an organism is surprising accommodating to this kind of mutation. Billions of years of natural selection has rewarded animals with a genetic code that encourages this kind of adaptive morphing.
Typically genetic mutations are minor, taking thousands of years to accumulate into perceivable change within a population. And most genetic mutations are neutral, providing no significant advantage or disadvantage to the organism. These mutations can occur in unused genetic stretches, or they can occur within protein coding genes without actually affecting the resulting amino acid chain. Occasionally though the mutation is much more severe, particularly when the genetic code is significantly damaged due to environmental pollution, radiation, or disease. The most noticeable mutations will occur due to genetic mutations of controlling genes (those which activate and deactivate other genes). In rare cases one slight change to the DNA controlling embryo development can alter how cells divide, and create extra, fully functioning appendages using the genetic code of existing anatomy. Sometimes all it takes is a minute change in the rate of cell replication at a key point in the development process. While the outcome for severe genetic mutations are typically negative, it also demonstrates the underlying logic of our bodies, and the process by which organisms can evolve. It shows how evolution can proceed either through incremental steps, and occasionally through riskier leaps. This kind of random experimentation is key to the process of natural selection. Without some accommodation for variation build into our genetic formula, evolution wouldn't be possible.
The mythology of the Abrahamic religions have taught that humans weren't just above the animals, but completely separate & distinct, made in the image of God (who in turn was thought to resemble man). It's understandable that being confronted the evidence for Homo sapiens' common ancestry with other species would be jarring to those who took these myths as infallible. Perhaps it could instead be a humbling notion, one that may change our appreciation for the life that evolved along side of us? Regardless of how we interpret what we now know, learning this evolutionary history places us within the larger context of an unfolding web of life. For the most part this evolutionary process is a beautiful system, although at times, such as with predation and extinction, it can be very ugly too. But from this process of natural selection, across a length of time so vast that it's difficult to fathom, consciousness evolved.