As we review the fossil evidence for early hominins, keep in mind the importance of identifying derived traits Ancestral traits are traits that have

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2 As we review the fossil evidence for early hominins, keep in mind the importance of identifying derived traits Ancestral traits are traits that have not changed from the earlier ancestral form Derived traits are traits that have changed or have been modified from earlier ancestral form Ancestral traits are those inherited from distant ancestors. Derived traits are those that more recently appeared and gave rise to a newly formed lineage.

3 What is ancestral or derived is relative to what lineage an organism is on E.g. Mammary Glands Ancestral trait of all mammals Derived trait of some Vertebrates

4 Derived traits mark a divergence separation from earlier ancestors and our diagnostic of a divergence between genera or species For example, bipedalism is a derived trait, a modification or change that characterizes a new lineage/branch Bipedalism is diagnostic of the divergence between bipedal and non-bipedal apes

5 Early hominins also had many ancestral traits that were unchanged and inherited from earlier nonbipedal ancestors This mix of ancestral and derived traits in early hominins explains why early hominins look so apelike When evidence of bipedalism is found in the primate fossil record, the species is considered an ancestor of humans

6 Ancestral Characters/traits inherited from a remote ancestor Not diagnostic of groups that diverged after the character appeared Derived (Modified) Traits that are modified (changed) from the ancestral condition are diagnostic of particular evolutionary lineages

7 A Closer Look at the Family Tree Early primate origins Miocene Fossil Hominoids Science of Paleoanthropology Early Hominin Tools Paleoanthropological Dating Methods

8 Members of the tribe Hominini Includes all bipedal hominoids back to the divergence with African great apes Share similar behaviors based on the biocultural nature of human evolution Biology makes culture possible and that culture influences biology

9 Evolved from earlier primates that date back to 50 mya Are all from Africa May date to more than 6 mya Most became extinct But some of these apelike mammals could be human ancestors

10 4.5 bya (billion years ago)-earliest life on earth 3.5 bya-first microbes 2-1 bya-first prokaryotic (bacteria-like) life forms 533 mya-cambrian explosion Called because there was an explosion in Biodiversity Huge increase in numbers of species Adaptive radiation 210 mya-non-placental mammals mya-mesozoic Era (Age of reptiles/dinosaurs) 65 mya - present is the Cenozoic Era

11 Epochs: time periods within the Cenozoic era Defined by life forms Divided early, middle, and late Early is further back in time Late is closer to the present

12 Paleocene: mya Eocene: mya Oligocene: mya Miocene: 23-5mya Primary focus for primate fossils Pliocene: mya Pleistocene: 1.8 mya-present The term Plio-Pleistocene combines the two Usually in reference to human evolution Holocene: 10,000 ya present Not all scientists agree that the Holocene is a separate epoch and consider the current epoch the pleistocene

13 The extinction of dinosaurs 65 mya marks the end of the Mesozoic Era, the Age of Reptiles, and the beginning of the Cenozoic Era, the Age of Mammals

14 We are still in the Cenozoic Era This chart summarizes the fossil evidence and evolutionary forces of three of the epochs (Eocene, Oligocene, and Miocene) of the Cenozoic Era

15 Demise of dinosaurs 65 mya allowed early placental mammals to expand into new environments vacated by dinosaurs This adaptive radiation of mammals included the ancestors of modern-day primates Adaptation to new environments may have led to rise of early primates mya Radiating out from Early placental mammals - plesiadapiforms

16 The roots of the primate order go back to the beginnings of the placental mammal radiation at least 65 mya The earliest primates date to the Paleocene (65-56 mya) and diverge from quite early primitive placental mammals, called plesiadapiforms

17 Unlike Paleocene forms, Eocene fossils display distinctly derived primate features (changed from ancestral form) Looking at the whole array of Eocene primates, it is certain that they were: 1. primates, 2. widely distributed, and 3. mostly extinct by the end of the Eocene Two types of Eocene primates

18 1. Adapidae (Adapids) 88 species in 37 genera Lemur-like NOT lemurs, NOT lemur ancestors 1 lb+ (but not huge) Diurnal? Binocular vision Nails Increasing brain size Exist from here until ~8 mya 47 my record!

19 2. Omomyidae (Omomyids) 93 species in 41 genera Tarsier-like 0.5 lbs Smaller, but overlap with Adapids Nocturnal? Insectivorous Teeth Exist from here until ~25 mya 30 my record!

20 A vast number of fossil primates from the Eocene (56 33 mya) have been discovered and now total more than 200 recognized species.

21 North Africa fossils dating from the early Eocene (50 mya) and Egypt (37 mya) Some are probably similar to and may be ancestors of the lemurs and lorises Most complete early primate is Darwinius, from the Messel site in Germany, discovered in 2009 and dates to 47 mya - may have become extinct and not be ancestral to any later primates Controversial since very few scientists have had opportunity to examine

22 Darwinius, 47 mya, most complete early primate fossil ever found Does not appear to be ancestral to any later primate Potentially an ancestor to lemurs and lorises, but traits not confirmed

23 The Oligocene (33 23 mya) has yielded fossil remains of several species of early anthropoids.

24 A lot of our fossils from this period are teeth Fossilize well 2 Main Types Parapithecids , same as NW Monkey Not ancestors Propliopithecids , same as OW Monkey

25 Environment Very different environment from today Continental plates have shifted

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27 Artist illustration of what early Oligocene primates might have looked like. They are believed to be similar in appearance to the Strepsirrhini (lemurs and lorises)

28 By the early Oligocene (33mya), continental drift had separated the New World from the Old World Late in the Eocene or very early in the Oligocene, the first anthropoids may have arose in Africa and reached South America by rafting over the water on drifting chunks of vegetation The ancestry of New and Old World monkeys was separate after 35 mya

29 Apidium Primitive dental arrangement suggests near or before evolutionary divergence of Old and New World anthropoids Small, squirrel-like, arboreal quadruped, fruit and seed eating, adept at leaping and springing

30 Aegyptopithecus Largest of Fayum anthropoids, about the size of a modern howler monkey (13-18 lbs) Short-limbed, slow-moving Bridges the gap between Eocene fossils and succeeding Miocene hominoids Aegyptopithecus skull Howler Monkey skull

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32 We have African, European and Asian forms of Miocene hominoids

33 1. African forms (23 14 mya) These hominoids are, in many ways, primitive or ancestral Proconsul has monkey and ape characteristics 2. European forms (16 11 mya) Most are quite derived (changed from ancestral form) Dryopithecus Ouranopithecus 3. Asian forms (16 7 mya) The largest and most varied group - most are highly derived (changed from ancestral form) Sivapithecus

34 Ape like features Did not have tail Shoulders and elbows like apes Skull is large relative to overall size Teeth like hominoid, shape of lower premolar Monkey-like features Limb proportions more like monkey s, arms and legs same size mya - Classified as an early hominoid because overall structure is more like an ape than a monkey, but still retains primitive traits or monkey-like features.

35 Dates back early Miocene 12 mya First species found in 1856 in France Ape and monkey characteristics

36 Teeth are very diagnostic for ancestry Survive due to enamel Enamel thickness is diagnostic Human lineage all has very thick enamel Living African apes, have (relatively) very thin enamel

37 Dental arcades (remember!) Human dental arcade is parabolic Other Apes dental arcade is U-shaped Both fossil/modern

38 Date back to 12.5 mya Previously known as Ramapithecus Pakistan, 1970s Parabolic-ish? dental arcade Thick enamel Was thought to be possible hominid, but debated? We learned: ASIAN apes also have thick enamel Only African apes have thin enamel Ramapithecus, renamed Sivapithecus Is orangutan (not human) ancestor

39 Shape of skull, teeth, and oval shaped eye sockets and eyes set close together are similar to modern-day orangutans Sivapithecus (L) odern orangutan (R)

40 Comparison of a modern chimpanzee (left), Sivapithecus (middle), and a modern orangutan (right) Both Sivapithecus and the orangutan exhibit a dished face, broad cheekbones, and projecting upper jaw and incisors

41 Miocene Apes in China 1. Lufengpithecus Miocene ape from China Find all the way through Pliocene (1 mya) Fossil is squashed flat Oldest consists of single mandible (jaw)

42 Miocene Apes in China 1. Lufengpithecus 2. Gigantopithecus 2 Kinds: A)Gigantopithecus bilaspurensis 8-10 mya India/China B) Gigantopithecus blacki

43 Gigantopithecus blacki Largest primate that ever lived Estimated to be ~600 lbs Separated from G. bilaspurensis by 7 million years ago Pictured: Far left: G. blacki Middle: Modern Gorilla Far Right: Modern Human

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45 Very few hominoid species (apes and humans) remain Even if Miocene fossils have some characteristics that resemble living orangutans or other apes, we cannot assume the fossil forms are closely related to living species We should assume that most lines vanish without descendants We can, however, note possibilities and patterns of change over time New fossil finds are constantly giving us more information about these early hominins

46 Date Hominoid Significance 55 8 mya Adapidae (Adapids) Eocene - Lemur-like, but NOT lemurs, NOT lemur ancestors 1 lb+ (but not huge); Diurnal? Binocular vision, Nails, Increasing brain size mya Omomyidae (Omomyids) Eocene - Tarsier-like 0.5 lbs; Smaller, but overlap with Adapids Nocturnal? Insectivorous mya Apidium Oligocene Primates from Fayum Primitive dental arrangement suggests near or before evolutionary divergence of Old and New World anthropoids Small, squirrel-like, arboreal quadruped, fruit and seed eating, adept at leaping and springing mya Aegyptopithecus Oligocene - Largest of Fayum anthropoids, about the size of a modern howler monkey (13-18 lbs) Short-limbed, slow-moving mya Proconsul heseloni Miocene - Nicknamed the Dental Ape. Classified as an early hominoid because overall structure is more like an ape than a monkey, but still retains primitive traits or monkey-like features 12 mya Dryopithecus Miocene - Ape and monkey characteristics 12.5 mya Sivapithecus Miocene - Previously known as Ramapithecus Was thought to be possible hominin ancestor, but later discovered to be orangutan ancestor 23 1 mya Lufengpithecus Miocene ape from China. Found right into the Pliocene 9 mya 100 kya Gigantopithecus blacki Largest primate that ever lived Estimated to be ~600 lbs Spanned the Miocene into the Pliocene all the way to the Pleistocene in China, India, Vietnam

47 Pre-Australopithecines ( mya) Sahelanthropus Tchadensis Orrorin tugenensis Ardipithecus ramidus ( Ardi ) Australopithecines ( mya) A. anamnesis A. afarensis A. platyops A. africanus Early Homo (2.5 + mya to present) H. habilis ( Handy Man ) H. rudolfensis ( Rudy ) H. erectus (ergaster/antecessor) A. aethiopicus A. boisei A. rubustus A. ghari A. sediba H. heidelbergensis H. neanderthalensis H. floresiensis ( Hobbit ) H. sapiens

48 First bipedal hominins appeared in Africa 6-7 mya Marks divergence of hominins and African apes Derived trait bipedal Ancestral traits - apelike teeth, ape-sized brains

49 East African sites along the Great Rift Valley where more than 2,000 hominin fossils have been found

50 The earliest possible hominins based on derived trait of bipedalism Sahelanthropus Tchadensis Orrorin tugenensis Ardipithecus ramidus

51 Three possible Pre-Australopithecine candidates Sahelanthropus (oldest, nearly 7 Mya) found 2002 no evidence so far of bipedalism Orrorin tugenesis (6 Mya) found 2001 potential evidence of early hominin bipedal trait Ardipithecus ramidus ( Mya) found 1995 toe bone indicates evidence of bipedalism All three are more apelike than human, but possible evidence of bipedalism places them on human ancestral line

52 The earliest bipedal hominins earliest possible human ancestors. Living at about the same time. Sahelanthropus tchadensis was found in Chad. Orrorin and Ardipithecus were found in Kenya and Ethiopia. (Scientific American, 8/25/2003)

53 Earliest hominin fossil found in Chad 1600 miles from Eastern Rift Valley Bipedal? Foramen magnum says yes Has unusual combination of traits: Ancestral apelike features Small braincase (350 cc) Massive build Huge brow ridges supraorbital tori Sagittal Crest Derived features (more human-like) Smallish vertical face teeth very unlike that of apes face not protruding reduced, non-honing canine teeth

54 Compare an Eocene primate and modern human skull The position of the foramen magnum indicates the angle of the spinal column to the head and indicates whether the body is habitually horizontal (quadrupedal like most primates) or vertical (bipedal like humans)

55 Contrast Sahelanthropus with Chimpanzee and Modern Human Notice: Teeth - canines Face protrusion

56 6 mya Found 2001 Western Kenya Fragmentary Bipedal femur, non-honing canines Curved fingers (ape-like) Smaller teeth than australopithecines Thin enamel

57 Ardipithecus (2 types) A) Ardipithecus kadabba B) Ardipithecus ramidus Ardi mya Middle Awash, Ethiopia ~ 4 ft tall, 110 pounds Non-honing dentition Primitive polishing Only hominin with thin enamel Brain size: cc

58 Ardipithecus ramidus Hominid who walked bipedally 4.4 mya ape-like dentition bipedal locomotion overall hominid-like skeleton small cheek teeth with thin enamel and large canines arm bones are hominid-like foramen magnum indicates bipedalism

59 Genus: Australopithecus mya Southern Ape: derived from the Latin word australo meaning southern and the Greek word pithecus meaning ape.

60 Small size Small brain, small canines Large premolars and molars Bipedal capability - yes 2 types: Gracile Robust Difference: Bone density, muscle markings, bone size, skull size, teeth strength, etc. Some give the robust their own genus: Paranthropus Your book does, but by no means agreed upon and I m afraid I am once again going to disagree with your book

61 Gracile Robust

62 1. Australopithecus anamensis ~4 mya Lake Turkana, Kenya/Ethiopia Similar to Ardipithecus Ancestral Traits: Large canines U-shaped dental arcade Ape-like premolars Appears transitional between apes and humans (below) The word anamensis is from anam ; Turkana word meaning lake and is a reference to the ancient lake-side environment once inhabited by this species.

63 2. Australopithecus afarensis mya Laetoli (Kenya) and Hadar (Ethiopia) Best known Australopithecine Hundreds of fossils including: Laetoli footprints The famous Lucy Dikika child Bipedal, but hands and feet suggest partially arboreal Speech? More varied diet Brain: 430 cc Various (forests, woodlands, open country)

64 Most famous A. afarensis specimen 40% complete skeleton Human ancestor Dated 3.2 mya 3 5 tall 70 lbs

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66 Walking with Lucy

67 3. Australopithecus platyops AKA: Kenyanthropus platyops 3.5 mya Same time as A. afarensis Lake Turkana Known for: Unusually flat face Woodlands

68 4. Australopithecus africanus 3-2 mya South Africa Robust Australopithecine Gave rise to robustus? Predecessors? Height, size similar to A. afarensis Larger teeth Famous ones: Taung Child Mrs. Ples Find A. africanus and A. robustus at the same time and place Brain: 450 cc Hint: Do NOT confuse africanus with afarensis!

69 5. Australopithecus aethiopicus 2.5 mya East Africa Known as: Black Skull Ultimate in robusticity Chewing MACHINE! Huge posterior teeth Small anterior teeth Thick enamel Related to A. boisei? Brain: 410 cc Aethiopicus is Latin for Ethiopian

70 6. Australopithecus boisei (AKA Paranthropus boisei) mya One abs. dated at 1.75 mya Potassium-argon East Africa Hyper-robust Nicknamed Zinj Or Nutcracker Man Originally Zinjanthropus boisei Brain: 510 cc

71 7. Australopithecus robustus (AKA Paranthropus robustus) 2 mya South Africa Descendent from A. africanus? Larger, more pitted teeth Robust Sagittal keel, splayed cheekbones, big brow ridges Chewing machine! Brain: 530 cc

72 A. aethiopicus, A. boisei, A. africanus, A. robustus all have: Large faces Massive muscle attachments (up to sagittal crest) Small front teeth Enormous premolars and molars

73 EXTINCTION OF ROBUSTS? Robusts persist for ~ 1.3 million years ( mya) World-wide climate grows colder during this time Grasslands dry out Change may have occurred too quickly for robusts to adapt Completely disappear from the fossil record

74 8. Australopithecus garhi 2.5 mya Middle Awash, Ethiopia Larger teeth than early Australopithecines Face: Primitive projection Name means SURPRISE! Surprising because: Human-like ratio of arms to legs Direct ancestor to Homo? Body gracile, robust-like teeth Hyperdont gracile ( Big teeth gracile ) Evidence for tool use Oldowan? Brain: 450 cc

75 9. Australopithecus sediba Name means source Fairly new find 2008 South Africa Two specimens in holotype: Juvenile male, adult female Brain:~450 cc

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77 9. Australopithecus sediba Dates ~2 mya More recent than A. afarensis Transition between A. afarensis and H. habilis/h. erectus Clear bipedal pelvis But long arms Retaining arboreal trait Very dexterous hand tool maker? A lot of Homo habilis specimens have been reexamined and reclassified into this species Comparison: A. Sediba 1, Lucy, A. sediba 2

78 Date Hominins mya Sahelanthropus tchadensis Oldest possible hominin ancestor 6.0 mya Orrorin tugenensis mya Ardipithecus ramidus Ardi 4.0 mya Australopithecus anamensis (gracile) 3.6 mya Australopithecus afarensis Lucy (gracile) 3.5 mya Australopithecus platyops Flat Face (gracile) 3.0 mya Australopithecus africanus Taung Child/Mrs. Ples (robust) 2.5 mya Australopithecus Boisei (robust) 2.5 mya Australopithecus aethiopicus (robust) 2.5 mya Australopithecus garhi Surprise! (gracile) 2.0 mya Australopithecus sediba Source (gracile) 2.0 mya Australopithecus robustus (robust)

79 Map of where the fossils were found.

80 Remember, this is our best idea of how the lineages play out but this changes with every new find!

81 Abundant Many well-preserved fossils Some with more complete skeletons or abundant body parts Well studied Growing consensus that A. afarensis is potentially the ancestor of all later hominins

82 Major features 1. They are all conclusively bipedal (although not necessarily identical to Homo in this regard) 2. They all have relatively small ape-size brains (at least compared to Homo) 3. They all have large teeth, particularly the back teeth, with thick to very thick enamel on the molars

83 Chimpanzee, quadruped, - feet far apart; femur comes straight out of the hip socket Lucy and the modern human feet close together, femur angles in from hip to knee

84 Footprints found in volcanic tuff Layer of ash from volcano that is cemented by rain Footprints of animals walking through the wet tuff before cemented Hominid footprints Fully bipedal 2 sets One bigger, one smaller Some argue there is even tinier footprints inside the bigger ones Dated absolutely to 3.6 mya Species that made the footprints A. afarensis

85 Laetoli Footprints, found in Tanzania

86 Hominin footprint from Laetoli, Tanzania, Africa Note the deep impression of the heel and the large toe (arrow) in line (adducted) with the other toes

87 Near Olduvai Gorge in Northern Tanzania 59 footprints of bipedal hominins (presumably afarensis) were found in a now hardened volcanic ash layer These A. Afarensis individuals walked in two close parallel tracks across wet volcanic dust at least 3.5 mya which hardened over time

88 The A. Afarensis footprints look almost like those of modern humans.

89 Bipedalism - ~6 mya Australopithecines Increase in brain size mya Genus Homo Development of stone tool technology mya Genus Homo

90 H. habilis H. rudolfensis H. erectus H. ergaster H. antecessor H. heidelbergensis H. neanderthalensis H. floresiensis H. sapiens

91 At least one species (maybe two) of early Homo was present in East Africa a little prior to 2.0 mya These developed in parallel with an australopithecine species These hominin lines lived contemporaneously for at least 1 million years after which australopithecines apparently disappeared forever

92 Compare brain capacity of Homo habilis to Australopithecines and later Homo erectus and Homo sapiens

93 Homo habilis handy man * African hominin fossils, Dates to million years ago Found in Olduvai Gorge in Tanzania. *Latin translation habilis: able to handle or skilled

94 Most primitive of all Homo long arms Short legs Short stature overall 4.5 ft fully grown Smaller face than Australopithecus

95 H. habilis was found with simple stone tools Tools are known as the Oldowan tradition

96 Simple chopping tools made by striking flakes off a rounded stone Gives it a rough cutting edge Capable of cutting animal flesh Requires considerable skill to make Made of lava and quartz

97 Used stone cores and small flakes Flakes for scraping wood, cutting meat, cutting grass stems Stone tools often found with butchered animal bones At Olduvai Gorge, stone tools and animal bones appear to have been brought from farther away

98 H. habilis H. rudolfensis H. erectus H. ergaster H. antecessor H. heidelbergensis H. neanderthalensis H. floresiensis H. sapiens

99 Lived in Africa about 1.9 mya 1972 Lake Turkana in Tanzania (then known as Lake Rudolf) Its cranial capacity is somewhat larger than H. habilis ( cc), but more similar to australopithecines in certain facial features Evolutionary relationships between species of early Homo are ambiguous

100 H. rudolfensis Brain: ~775 cc Cranial capacity is one of the main distinguishers between H. habilis and H. rudolfensis Homo size brain, but robust Australopithecine teeth (big brain, big teeth)

101 KNM ER 1470 (H. rudolfensis) KNM ER 1813 (H. habilis)

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103 In Rudy, see post-orbital expansion compared to Australopithecus Doubling of brain volume ~440 cc (average Australopithecine) to ~775 cc (H. rudolfensis) Less Prognathism (protruding jaw)

104 Which best represents our ancestor? H. habilis has smaller, human-like teeth But also a smaller, australopithecine brain H. rudolfensis has a bigger, human-like brain But also has larger, australopithecine teeth Big debate: which best represents our ancestor? Small Teeth or Big brain?

105 It is difficult to learn anything about the earliest stages of hominin cultural development before the manufacture of stone tools Archaeological record is almost exclusively limited to material culture We can assume that early hominins used tools made of perishable materials, like wood, animal parts, plants, etc., and displayed an array of cultural behaviors even if little evidence remains in the fossil record

106 Multidisciplinary approach: Geologists Survey to locate potential hominin sites Vertebrate paleontologists Survey geological beds containing faunal remains and look for favorable conditions Paleontologists Give approximate ages of fossil sites in the field using faunal sequences Archaeologists Get involved with sites that post-date 2.6 mya to look for material hominin traces - artifacts

107 Gave crucial adaptive advantages to hominins Earliest tools likely made of perishable materials, leave no trace in fossil record Earliest stone tools date to about 2.6 mya, small sharp flakes Hominin bipedalism would have made tools easier to transport

108 Earliest recognized stone tool culture Oldowan tools found primarily in Olduvai Gorge in Tanzania Excavations led by Mary and Louis Leakey

109 Relative dating methods tell you that something is older or younger than something else, usually gives a range of dates Chronometric (Absolute) dating methods are based on calendar years, gives specific dates

110 Stratigraphy based on the law of superposition, which states that a lower layer is older than a higher one Biostratigraphy based on regular changes seen in dentition and other anatomical structures in groups of pigs, rodents, and baboons Paleomagnetism based on the shifting of earth s geomagnetic pole; by examining magnetically charged particles

111 Stratigraphy The horse skulls on the left are arranged according to the dates of the geological strata/layer of earth.

112 Stratigraphy Law of Superposition Defined: In an undisturbed series of sediments, the younger layers are on top of the oldest ones Sediments have constant layering and build up over time The oldest ones got laid down later in time The more recent ones were laid down subsequently

113 Biostratigraphy Layers of the Earth Biostratigraphy Names each layer based on the organisms that are fossilized within E.g. Miocene: 23-5 mya

114 Based on Radioactive decay Radioactive isotopes of certain elements are unstable and decay to form isotopic variants Decay rates follow a predictable mathematical pattern Accurate Geological Clock

115 K/Ar Method Potassium Argon method used to date materials in the 5-1 mya range Potassium-40 has a half life of 1.25 million years Potassium isotopes produce Argon-40 Cannot date organic material, but good for dating rock containing fossils By comparing the amount of K-40 to Ar-40 in the volcanic rock, the date that the rock formed can be determined

116 U/Pb Method Uranium half life is 4.47 Billion years Produces lead Good for dating very old non-organic materials

117 Carbon-14 method Half life of carbon-14 is 5,730 years Used to date organic material extending back to 75,000 years; most accurate up to 40,000 years

118 Thermoluminescence Detects the luminescence of heat released from stone materials once they have been struck or altered Good for dating stone tools and ceramic artifacts

119 Combining dating methods If hominin remains are found between 2 layers of volcanic ash, one dating to 3.8 million years ago and the other dating to 3.2 million years ago, the hominin can be dated at between 3.8 and 3.2 million years ago.

120 Pattern in which physiological and behavioral systems evolve at different rates Each component that defines hominin evolution (dentition, locomotion, brain size, and toolmaking) have developed at quite different rates

121 In hominin evolution, bipedal locomotion is a defining characteristic Other features such as brain development and behavior become significant in later stages