The Origin of Mammals

24 “And God said, Let the earth bring forth the living creature after his kind, cattle, and creeping thing, and beast of the earth after his kind: and it was so.

25 And God made the beast of the earth after his kind, and cattle after their kind, and every thing that creepeth upon the earth after his kind: and God saw that it was good.”

Genesis 1:24-25

“All flesh is not the same flesh: but there is one kind of flesh of men, another flesh of beasts, another of fishes, and another of birds.” 

1 Corinthians 15:39

Naturalistic Worldview:

Naturalists believe that reptiles evolved into both birds and mammals.

Key Changes Needed:

1. The development of hair.

2. The development of a glandular system.

3. The development of the ability to regulate body heat.

4. A drastic change in their metabolic rate.

5. A drastic change in their growth rate.

6. The re-location of their limbs.

Similarities and Differences:


1. Both reptiles and mammals have vertebrates.

2. Both have a sophisticated nervous system.

3. Both have spinal cords.


1. Reproduction: Reptiles are mostly egg-laying creatures. Mammals give birth to a live young.

2. Body Coverings: Reptiles have tough scales and no glands. Mammals have furs or hair and have sweat glands.

3. Regulating Body Heat: Reptiles are cold-blooded and have no system to regulate body heat. Mammals can produce body heat.

4. Nursing young: Female reptiles lack mammary glands, and most species abandon their offspring soon after they hatch. All female mammals have mammary glands that produce milk, allowing them to nurse their young.

5. Teeth: Reptile teeth are uniform in shape, though they might vary in size. Reptiles' teeth grow and are replaced continually throughout their lives.

Mammals grow only two sets of teeth, with complex cheek teeth, and they do not continue to grow throughout their lives. Mammals have specialized teeth, such as canines for tearing through meat and molars for grinding food.

6. Metabolic Rate: Reptiles have a very low metabolic rate. Mammals have a high metabolic rate.

7. Hearts: Reptiles have three-chambered hearts with two ventricles and only one atrium. Mammals have a heart that consists of four chambers, two ventricles and two atria.

8. Growth Rate: Reptiles continue to grow throughout their life. Mammals reach a growth limit and stop growing.

9. Glandular System: Reptiles do not have a glandular system. Mammals have sweat glands.

10. Skulls: Reptiles have a skull with a small brain case. Mammals have a skull with an expanded brain case.

11. Ears: Reptiles have a single middle ear bone. Mammals have three middle ear bones.

12. Jaws: The lower jaw of reptiles is made up of multiple bones. The lower jaw of mammals consists of one bone that is firmly attached to the skull.

13. Pelvis: Reptiles pelvic bones are separate. Mammals pelvic bones are fused.

14. Limb Location: Reptiles have a sprawling gait with their limbs emerging horizontally from their body. Mammals have an upright stance with their limbs directly beneath their body.

Reptiles are extremely different from mammals. So much so, that you would just about have to redesign them from the inside out, to turn one into another. Those with a supernatural worldview believe God created them as separate "kinds," and intended for them to remain that way, except for speciation within their created "kind."