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The newest fossils will be in the layers closest to the surface.

A fossil with four limbs and digits would be the fossil of a more recent organism than a “standard” fish. Thus, the “standard” fish fossil would be in deeper layers, and the four-limbed fossil would be in layers above that fish fossil.

This fossil seems to be similar to modern-day bony fishes. It is possible to observe fins with rays, suggesting that this organism was aquatic and able to swim. There does not appear to be a distinct neck, consistent with a fish, and there could be a gill cover present, again consistent with an aquatic organism. This may have been a predatory fish, as there appear to be teeth present.

The hypothesis should not be rejected because of the absence of sea cucumber fossils from the fossil bed. Soft-bodied organisms (such as sea cucumbers) do not fossilize well, and so do not leave fossils in the fossil record.

As the oysters date from 100 million years ago, and the barnacles are found in the same layers as the oysters, the barnacles must be at least 100 million years old.

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Long and sturdy ribs (to help support the body) and pectoral fins that have wrists and can bear weight.

Transitional fossils are “intermediate” in that they are midpoints between two groups of organisms. They are important because they shed light on how groups of organisms were able to move into new habitats or develop new structures. Titkaalik represents the transition between aquatic bony fishes and land-dwelling tetrapods.

The first real tetrapod would likely have fully developed forelimbs and hind limbs, both of which would have jointed wrists and ankles and could bear weight. These fossils might also have more distinct jointed digits (both fingers and toes). These fossils would be younger (that is, more recent) than the Titkaalik fossils, as true tetrapods would be the descendents of Titkaalik.

While land represented a new opportunity, the oceans, lakes, rivers and streams still provided many opportunities for survival and reproduction, and fish that were well adapted to this environment continued to leave descendants.

The skeletal anatomy of a chicken wing and that of a human arm are very similar. All major bones are present in each, and in the same locations relative to other bones. In the human, the most distal bones (the tips of the digits) are longer and arranged in a way that permits fine manipulation of objects with hands and fingers. Birds do not need to carry out this fine-scale manipulation, and their wings are specialized for flying.

middle ear bones in humans; gills in adult bony fish

You could make pair-wise comparisons between the sequences, counting the number of nucleotide differences between them. The more differences there are, the less similar the sequences are. More-similar sequences suggest closer relationships, and less-similar sequences suggest more distant relationships. Ideally, you would compare a large number of genes before coming to a conclusion.

Humans and otters share a common ancestor, and therefore homologous bone structures, including five digits on the hands. Both humans and otters are dexterous with their hands, using them for a variety of important functions, so the continued presence of five digits would have been selected for over evolutionary time.

At early stages of development, both human and chicken embryos have a tail, so this feature will not allow you to distinguish between an early human embryo and an early chicken embryo.

As the genetic code is universal (with only a few exceptions), the same DNA sequence will encode the same amino acid sequence in both humans and bacteria.

Both proteins are encoded by gene X in each organism. The fact that the amino acid sequences differ between sea urchin and yeast means that the gene sequence of gene X differs between sea urchins and yeast. This is because sea urchins and yeast have each acquired mutations (that is, nucleotide changes) in the gene since they diverged from their last common ancestor. The greater the number of nucleotide differences, the longer it has been since the organisms split from their common ancestor, and the less closely they are related. The sequence of gene X from humans could be compared to the sequence from sea urchins and yeast, and the number of nucleotide differences could be used to establish relationships.